What's this stability bug issue?

If I go to change the thatch in the center to insulated inverted roof both center pieces come out and cannot be replaced. I can only replace the inverted with thatch pieces what kind of bug is this?

Stability on those pieces is 60% so they should be able to be upgraded with a higher tier. And how come it will only accept thatch at the material

I experienced this too.
I built a round based building, made the roof of thatch first but when I replaced the pieces with the higher tier ones the top collapsed and could not build it back.

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