If you open the steamvr menu after hitting the calibrate button to try and disable full body it will put you in 3 point vr correctly but does not get rid of the calibration balls and controllers. In addition it does not give you the seated/standing toggle.
EDIT
As I discuss the unsupported steamvr method I fear this report will get ignored. Let me explain it like this:
If for any reason vrchat is missing the required trackers to initiate the FBT calibration, after FBT was previously detected , it will enter 3PT tracking and this bug will occur. The steamvr example is just an easy reproducible method.