This happened with me as well when I added the Lime Sewing Machine into my home. My pets were at max slvl anyway but there was an empty space so I had wanted to put something in it and the slvl went down by one. None of my other decorations did this strangely enough though.

Edit: Played around and it’s changed. Lime sewing machine no longer does it but placed both purple safes and it did it (but it seems removing something fixed it... I removed the starter window. Specific items seem to be glitched)