Thanks, and for that Issue link. As you say, I expect it’s just nuking the cookie and everything related to it just disappears.
At least it’s on someone’s radar. :)
Edit: I sent a message to support, as suggested by slazer2au, and got a response pointing out that 30 days seems to be hard-coded into mlmym: https://github.com/rystaf/mlmym/blob/a518844b005179623d0d7a31f45966cd8a6b8a96/routes.go#L794
That and those settings all being stored client-side explains everything. Not sure why those choices were made, but now I know why.
Searched replies and was not disappointed. 😄
Also: phone, earphones and mask these days.