Unable to save mesh in Mesh Editor (UE 4.24.2)

I modified a duplicate of the floor mesh in the 4.24.2 empty ArchViz template, and saving wouldn’t work. A popup message said it couldn’t be saved to disk, and would only be saved for the current application session. I only changed lightmap resolution to 128, generate lightmap with destination channel to 1 from initial CH 0 from what I recall. Is there a setting I need to change so it saves? That was not happening in 4.23.1.