When exporting objects that have been mirrored to .fbx. The objects texture's appeared off sometimes not showing the mirrored side or the mirrored side having an odd texture. You can see this in practice when comparing the tombstones in my Halloween Special video (https://youtu.be/x05ARdrH3G4) to those exported to .fbx on Sketchfab (https://skfb.ly/6CC9n) from that video.
Hey, thank you for this submission. We can see the bug and will try to fix it asap!