Hi there! I just made an account so I can give my few cents on this topic by giving my recent experience.
It happens that today I got Megan from a nice person. Everything went and seems pretty normal so far.
This person was trying to force Megan to move out trough TT, wich of course, it ended happening and I went to invite her to my island without issues. I just only went to talk to Megan and I left, nothing more.
I came back to my island and checked the plot, and indeed, it was sold to Megan.
Because reasons, I time traveled just one day forward. Isabelle told me Megan arrived. I completely forgot to check if Megan was there in boxes, that day.
I came back in time to the actual date, and I found that Megan was strolling around my island, eating a sandwich; pretty normal, so I guess I didn't got a glitch after all.
I wanted to share this mostly because I tought that if you completely ignore the villager in his "moving in" boxes state, not even checking the house, may help in not getting the glitch at all. Another thing that I tought was probably time travelling 2-3 days after you got the plot sold, because at that point a new villager should be already stablished.
Another info that I would like to share in case it worths for research:
-The plot was completely new, it was recently installed. No villager was using it before.
-Playing in Southern Hemisphere
-I have a physical copy of the game.
Hope this helps for some research. I really hope this glitch gets fixed soon with the Earth Day update
It makes me feel weird to play the game now.
I'll try to update you if Megan gets weirdly glitched or something happens tomorrow, since I have read that some people get their villagers glitched even after they behave normally in their towns.