I had this issue, too - REALLY annoying. There's a script (from one of the mods), which is firing at the wrong time and 'causing the kick-out of the build mode. In your case its possible, that a ghost of the script was stored in the savegame, 'cause deleting mods does not remove all parts of scripts. So a script cleaner, which is detecting and removing orphan scripts would be VERY USEFUL. There was such a thing in Skyrim..... I never found out, which mod was causing this - but, as in your case, it fixed itself....... No idea.