Whats happening at least from my experience is that the stability is snapping to different pieces or not at all when the server/single player session restarts. I posted a topic about this a few weeks ago where I was noticing that when you upgraded peices it would snap to a different stability from the top down.
Its a really frustrating bug that messes with all base peices from sandstone to stormclass. It made me walk away from the game because I had my entire lighthouse I was making collapse on me.because the top peices changed stability when I was upgrading from bottom up.
Basically don’t upgrade base peices, just build them from the ground up with your choses material.