最近のメールシステムでは不審な動きをしないか、受信時に内部的にクリックするような仕組みがあると思います。このような仕組みが働いて開封された結果となっているのではないでしょうか？
Taking Extra Steps: Evaluating Bot Clicks in Email Opens
If you’re also triggering automations off email clicks, this is a good opportunity to check your recipient lists for email security tools. These tools will pre-click every link in an email before delivering it to the recipient, who may then also click the links that are most interesting to them.
You may have noticed this show up in your email reports or in a prospect report, where it might look like a recipient clicked every link in your email — including your social media links and privacy policies — in under a minute. Impressive, if it were true!