UPDATE: its been exactly a week since this happened, and i still cannot replace these missing floor pieces. the situation, however, has certainly developed:
in the image below you see a room with a giant hole in the floor. these are the floor pieces in which the game says “failed to place building”. even though there is ample stability and i built this castle almost a year ago and there has always been a floor here.
youll notice that most of the floor pieces are argossean and some of them are nemedian. i had an idea that maybe if i replace the entire floor with a different build set, and then make it nemedian again, it might fix it. turns out that the floor pieces which are still nemedian were not replaceable. trying to turn them into argossean, i got the same “failed to place building” message. im willing to wager that if i delete those nemedian floors, i wont be able to fill them back in, thus making this irrepairable hole even larger.
and as it turns out, i was completely spot on. all of the nemedian floors that i couldnt replace with argossean have been deleted, and cannot, infact, be replaced:

i hesitantly deleted the entire floor, hoping to resolve what ever unfathomable issue this is that is causing me to lose an entire room in my base. this is as far as i am able to get before the game tells me “failed to place building”:
i play on a private pve server pretty much alone. i have 2 other friends i occasionally play with on here. 80% of my time spent playing this game is spent on building. i enjoy building gigantic 15th century style castles.
this absurd, unimaginable bug that has been going on for exactly a week is costing me an entire room in a structure thats connected to a greater complex which is intended to be a massive showcase build that ive been working on for almost 12 months. and now ive lost a room because… ? i really dont understand. this is the result of accidentally deleting a single wall.
how am i supposed to finish this build? do i have to delete the top half of the tower that this room is located in? do i have to delete the entire tower? do i have to delete the entire castle its connected to?? how far will this go? it seems the more that gets removed, the more that cant be replaced. this is ruining the entire build. this is an extremely poor way to begin age of war chapter 4 which ive been looking forward to for most of chapter 3.
i will continue to post on this thread and keep it relevant for as long as it takes until i get some legitimate answers/solutions/response from the devs on what to do about this because this is absolutely ridiculous and should not even be a thing thats happening. for me, based on the way i play this game, and the amount of time iv spent on this showcase build thats STILL a work in progress, this is absolutely game breaking.