Oh yeah, I've seen this too but only when I have multiple Lemmy instances open. When I posted "Death by user count" it told me I posted it to lemmy.einval.net through lemmy.ml's instance. I had to shift+ctrl+r to get it to show the correct pathway. Its definitely weird that session data is leaking around between logins. You'd think each cookie/session/socket would be totally unique.
I saw they've been working on ditching websockets in favor of a pure REST API. Hopefully this will stop when they release v18.0.