I am seeing the same behavior with an email very similar in content that I have sent to two different people. One person replied but it shows the subject line of the other conversation.
It MAY have to do with the pattern matching done to group emails into conversations. I have posted a bug report but haven’t gotten a reply. I have specifically said that I’m sure I could fix this by myself but again, I need someone to tell me where that matching logic sits as it appears to be undocumented.