TL;DR

Array 型の要素が全て ASCII のみの文字列なら問題は無い
find しただけのインスタンスなら非 ASCII 文字を含んでいても問題は無い
save した後のインスタンスをエンコーディングが必要な処理に通すと問題発生(Encoding::UndefinedConversionError)
未解決
- save 後に reload すれば回避できる、という逃げ道はあるけど……
- そもそも Array 型を使わなければ良い、という話ではあるけれど……

再現

$ rails new test-pg-array -d postgresql --api -T -C
$ cd test-pg-array
$ rails g model PgArray data:text
$ vi db/migrate/20160720125308_create_pg_arrays.rb
$ cat db/migrate/20160720125308_create_pg_arrays.rb
class CreatePgArrays < ActiveRecord::Migration[5.0]
  def change
    create_table :pg_arrays do |t|
      t.text :data, array: true

      t.timestamps
    end
  end
end
$ rails db:create db:migrate
$ rails r 'PgArray.create!(data: %w(a あ)).to_json'
/Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/core_ext/object/json.rb:34:in `encode': "\xE3" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/core_ext/object/json.rb:34:in `to_json'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/core_ext/object/json.rb:34:in `to_json'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/json/encoding.rb:55:in `to_json'
        from /Users/koshigoe/.rbenv/versions/2.3.1/lib/ruby/2.3.0/json/common.rb:224:in `generate'
        from /Users/koshigoe/.rbenv/versions/2.3.1/lib/ruby/2.3.0/json/common.rb:224:in `generate'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/json/encoding.rb:99:in `stringify'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/json/encoding.rb:33:in `encode'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/json/encoding.rb:20:in `encode'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/core_ext/object/json.rb:37:in `to_json'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/railties-5.0.0/lib/rails/commands/runner.rb:63:in `<top (required)>'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/railties-5.0.0/lib/rails/commands/runner.rb:63:in `eval'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/railties-5.0.0/lib/rails/commands/runner.rb:63:in `<top (required)>'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:293:in `require'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:293:in `block in require'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:259:in `load_dependency'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:293:in `require'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/railties-5.0.0/lib/rails/commands/commands_tasks.rb:138:in `require_command!'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/railties-5.0.0/lib/rails/commands/commands_tasks.rb:104:in `runner'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/railties-5.0.0/lib/rails/commands/commands_tasks.rb:49:in `run_command!'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/railties-5.0.0/lib/rails/commands.rb:18:in `<top (required)>'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:293:in `require'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:293:in `block in require'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:259:in `load_dependency'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:293:in `require'
        from /Users/koshigoe/tmp/test-pg-array/bin/rails:9:in `<top (required)>'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:287:in `load'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:287:in `block in load'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:259:in `load_dependency'
        from /Users/koshigoe/tmp/test-pg-array/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/dependencies.rb:287:in `load'
        from /Users/koshigoe/.rbenv/versions/2.3.1/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from /Users/koshigoe/.rbenv/versions/2.3.1/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from -e:1:in `<main>'
$ rails r 'puts PgArray.first.to_json'
{"id":1,"data":["a","あ"],"created_at":"2016-07-20T12:55:46.951Z","updated_at":"2016-07-20T12:55:46.951Z"}
$ rails r 'puts PgArray.create!(data: %w(a あ)).reload.to_json'
{"id":3,"data":["a","あ"],"created_at":"2016-07-20T13:17:38.298Z","updated_at":"2016-07-20T13:17:38.298Z"}

メモ

pg gem を使った Array のエンコード、デコードの挙動確認

(001): >> encoder = PG::TextEncoder::Array.new(name: 'text[]', delimiter: ',')
=> #<PG::TextEncoder::Array:0x007feb5d0fd368 "text[]"  elements_type=nil needs quotation>
(002): >> encoded = encoder.encode([%w(a あ)])
=> "{{a,\xE3\x81\x82}}"
(003): >> decoder = PG::TextDecoder::Array.new(name: 'text[]', delimiter: ',')
=> #<PG::TextDecoder::Array:0x007feb5c277848 "text[]"  elements_type=nil needs quotation>
(004): >> decoded = decoder.decode(encoded)
=> [["a", "\xE3\x81\x82"]]

関連