If you disable full body, by for example opening the steamvr menu, the seated/standing play toggle is not available in 3 point vr.
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 the seated/standing toggle that works in the live version of vrchat is missing because the calibrate button has replaced it even though you are no longer in 4/6PT tracking.
I would consider this a UI bug because 3PT tracking is officially supported in vrchat and has a functioning seated/standing toggle. Needing to restart the game just to get a UI button back sounds like a bug to me or potentially a poorly implemented calibrate button that does not return to the seated/standing toggle when FBT is not detected.
You can go from an officially supported 3PT tracking session to the game correctly identifying 4/6PT tracking that then switches the UI, but the current beta does not switch the UI back to the original 3PT tracking session when FBT is no longer detected. This is the whole reason I posted this report, no matter how minor it is, as I noticed functionality from the 3PT session just goes missing in favour of a game restart instead of the UI just silently changing back.