For those just tuning in, here is the official current housing guide (last updated Jan 2023)
https://na.finalfantasyxiv.com/lodes.../housing_land/
The relevant bit here is this:
Maintaining a free company plot means that the character who originally purchased the plot exists in the free company who currently uses the plot.
There are cases where a character may exist in a free company, purchase a plot, and then leave the free company. This feature ensures that they may be able to purchase a plot for another free company down the road.
This may indeed result in a player having characters/alts on the same service account, each of whom occupies a free company that has a housing plot. However, the purchaser restriction still exists. Work around that as you need to. There are hundreds of FC-only plots available to anyone who desires one across various worlds.