Precise snap creates gap? (Import problem?)

Hello, I have really wierd problem. When I import mesh (400x400cm plane) from Cinema 4D (import from Blender shows only first warning) it show warnings.

No smoothing group information was found in this FBX scene.

Face material index incosistency - forcing to 0

Problem apears when i create rooms with this mesh. When I walk through these rooms in corners flashes skybox… Like if there were gaps.
I don’t know why, but it changes transform rotation from -90° 0° 180° to this -89.999939° 0.000007° 179.999634° (It sometimes holds right value, but flashes reminds)


What is wrong?

Thanks for reply