I am having a very similar issue. I have tried multiple different things to eliminate other possibilities but every time I try to delete a wall in build mode the game freezes and I can't do anything ...
Ideas
I started having a similar issue intermittently after the 25th birthday update, and now it is constantly happening with the new patch update after Businesses and Hobbies came out. If I delete a wall segment in build mode, I cannot "undo" or build or move anything. And saving or changing lots after this happens just results in the game being stuck loading. I have had to force close every time.