I don't think this is unique to 16.5, I've seen this issue quite a few times after an iOS upgrade. To add insult to injury in my case I have never even had any email accounts set up on my iPhone, have never once opened the Mail app, and I have background refresh globally set to off!
In some cases I've found that the Mail background activity stops after a few days, sometimes quite a few days as in more like a week before it stops appearing as an app consuming resources in the Battery settings section. One other time, following a suggestion by someone on this forum (thanks whoever you were!) I did a network reset - which is a lot less dramatic/inconvenient than doing a total factory reset - and that fixed the problem for me on that occasion.