Its a bit of hidden knowledge and also quite sad but paradox has two kind of ship sets in the agme. One with ship section models that are not used and one of the ones we know. If a mod however utilise these models with your shipset there will be errors. I think this can be easly circumvented by just copy-past your existing shipsets and extended them to the three missing ones.
This effects Battlehip sections all of the "Breaker-class".

Also i think there where station and fortress models in the game that can be used by mods and these are also missing from this set.

All this can be easly reproduced by istalling the Better Combat mod that adds back all these sections and models.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2390168133

To be more precise thiere is a "lost" or "hidden" model typ of a Defense Station that needs to be added to this set, at least in a functional sens.

Also the Mega structure construction site also misses its model.