Yeah screenshots would be great. Dug through the post history a bit and only found one other "It happened again" post, no ss, gave up on locating the original after a few pages. I did find this one where OP might have clicked a potential suspicious link, which could be related.

Also curious to know if it's just a visual glitch or if OP actually has full access to another user's account, like inbox messages, ability to make posts as them, upvote/downvote, etc.

OP, I recommend downloading a screen recording app and recording your logins on the off-chance it does happen again.