This is a "fun" one.
Mailman3 along with lots of other new things, now has a decent bounce processor which means now bounce rules are actually being enforced now. So lots of disabled emails (departed staff, AOL/Yahoo spammers), trigger a bounce reaction and would be removed from your mailing list. If the owner (you) is not removed due to excessive bouncing, just ignore the emails. I assume it'll take months until we fully clean up the disabled emails.
The problem of the owner being removed due to excessive bouncing is another story though. Basically, spammers send virus or spam to -owners@ or it gets held for moderation, which means mailman happily forward the email to the owner. Your mail server (Gmail, etc.) sees the virus/spam and bounces it. Mailman thinks "Oh it address is disabled" and once you bounce more than a certain threshold, it'll disable or remove you. We should work with upstream to fix this but in the meantime, you can play with bounce processing settings
https://lists.wikimedia.org/postorius/lists/lgbt.lists.wikimedia.org/settings/bounce_processing but don't worry, it won't remove any members except owners.
HTH