As far as I can tell this only affects one particular avatar, and only when using --legacy-fbt-calibrate.
The first calibration after startup goes just fine, but the second one onward causes the avatar to float, so you need to playspace move to calibrate well. Toggling avatar measurement works around the issue, but subsequent calibrations with the avatar still cause it to float.
Doing reset avatar does not seem to help.
The problem does not manifest when using legacy IK.
There is a public sample of the avatar that also exhibits the same symptom, uploaded by its author at avatar ID: https://vrchat.com/home/avatar/avtr_9009b6ee-c483-4144-9e93-81b93803f11c
It is SDK2, so testing whether reset avatar does something is not possible, however.
Here is a video of me explaining the problem:
Here is me showing absence (or at least not being able to detect) this bug when using non-legacy calibrate: