idk where i read it but someone said that it wasn't supposed to happen, but then my last one was just taken by someone i didnt invite just now? is this part of the glitch too??
Not a glitch. 10th spot will get occupied by a random villager if you do not invite anyone to your island. From my experience, the plot will be empty for around 1 to 4 days until the game fills it up.
I'm pretty sure that happened to me too with Wolfgang. he was my 10th but he definitely wasn't from anyone's void or island. it seems normal tho, I wouldn't think it has anything to do with the glitch.
It definitely does fill up on its own.
I left mine empty on purpose (I was out of Nook Miles and too tired to grind) and it filled out with a random in two days.
I've only played online with one person so far and they never had this villager so it's not related to accidentally adopting someone.