You’re old email was invalid, which was why the system changed it to what you are seeing now. It was not changed by a hacker, but it was done for your own safety. Your old email, which does not exist, could easily be created and used to recover your account, but it’s impossible to create @u3658800@bounces.invalid as an email. Check the administrator statement quoted below for more.
This is something we recently started doing. If email sent to your email address bounces with an error message like, "This email address doesn't exist", then your email may eventually be changed to u...@bounces.invalid. (It's not possible for users to change their email address to something ending in .invalid, so this can only be an administrative change.) Because your old email didn't exist, somebody could've registered your non-existent email address and used that to steal your account.
I didn't particularly intend for the trust warning to appear for these automatic changes, but it's a niche situation and a bit difficult to fix, so I probably won't fix this unless several other people complain. It only lasts 30 days, after all.
Thank you for your prompt response, that has resolved my query. I wasn't aware of that information before. I'll be locking this thread now as the issue has been addressed.