I suspect maybe there was an issue with "the floor is lava revival" as it also requires building to be on a foundation... but I can't be certain. I uninstalled both mods, tried reinstalling just the foundations mod, enabling it, starting a game, disabling it, starting a game... nothing seems to work to allow me to play a vanilla game again? Have you heard of this before I do a reinstall?

Reinstalling didn't help, but deleting the %AppData%\Roaming\Factorio folder did the trick. Now to track down what file it modified so I can play again on my saves I backed up.

edit: it was something in the mods folder.. maybe mod-settings.dat had some kind of issue removing a setting? I'm not expert enough to know. But there you go.