Lot type changed on its own? Can't change back.

I'm a bit stumped. I logged into my game this afternoon, and came to find my legacy family's lot has changed itself from a residential lot to a park.. Completely on its own. On top of that, I have no option to change it back. It tells me the lot type can't be changed 'while its owned'. Any ideas as to what has caused this? There's a family of 8 living on this lot, which has been in use by this family for maybe a year, if not longer. 

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