"Please HELP" Export ArchiCAD file to Twinmotion -> slab element gets bumpy

Hello everybody!

I am using RIKCAD and I just exported a file to Twinmotion… and this thing happened ↓


RIKCAD is Japanese CAD and its engine is based on ArchiCAD so please help me if you know how to solve this…

The slab element is originally just flat. No elevation difference whatsoever.

I used polyline to draw this outline of the pond and I’m thinking maybe this is why the bump appeared…???

Please help me if you know anything about this problem.