I just had this same thing happen after the Update of June 2nd. It was the lot across from the Goth household called Hollow Slough or however that is spelled. It was the only lot ever played in that particular save and it had never been anything else other than a residence with a scratch built house on it. I had loaded the save and decided to put some new windows in it and entered build mode. First thing I noticed was I couldn't afford anything at all even though the household had over §38K. Then I noticed the business funds was §0 but my household doesn't own a business and the lot type was set to Generic. I could not change the lot type because it said it was a "Special Venue".
So at this point I moved my household out of that lot and into the blank lot across from the Pancake household. Then I went back to the original lot and saved it to my Gallery so I could plop it down on the new blank lot my Sims had moved to. In the Gallery it now said the original lot creator was someone else even though I built this one from the ground up and not even one room was from the Gallery. Anyway, I plopped it down onto the new blank lot my Sims had moved to and suddenly the lot type changed to Generic even though it said Residential in the Gallery.
I moved my household AGAIN to another lot and was then able to go set the lot type of the house I had just plopped back to residential. However, it still wouldn't let me change the original lot (across from the Goths) back. I then bulldozed the lot and it still says it is a Special Venue and I can't change it away from Generic. At this point I moved my household back into the lot across from the Pancakes and it stayed residential. They now live in their old house but it is now on a different lot and I can't seem to do anything with the original lot. It just sits there, bulldozed and Generic.
I have not used any CC and currently have all the Packs except of course the new Restaurant one that is coming out soon. I will try the workaround mentioned in this thread about buying it as a business and selling it to see if that works. Later...