Yup, that’s it! Building on festival lots and/or with seasonal lot markers on the lot will cause some objects to become transparent as you manually switch between different modes (construction/summer/fall/winter/spring) or go back and forth from edit town. It's been awhile since I've built with those that I had forgotten. So yes, definitely nothing to do with tomb-building techniques, not a glitch, and the game is functioning properly with these.
If you don’t want any objects to change with the seasons (or for anything to be transparent while you’re building), change your lot assignment to something else and remove the seasonal lot marker.
Glad your other issues have been figured out also.
