bd_
I believe I've found an issue in which synchronous ownership transfer fails, resulting in synced variables not being transmitted. The repro world is at https://vrchat.com/home/world/wrld_9ec88351-c7ad-4fbb-8d3d-766d17e9752b
Repro steps:
  1. Join two users
  2. As non-master, pick up a brush (behind the easels) and touch the canvas on the easel
The text below the easel shows canvas debug information; of interest here is "O" showing current owner, and G, which is a synced variable which should transition from 6 to 7 when first touched with a brush. However, on master, this value does not change, and instead a "Non-owner 2 attempted to send serialization data for Texture Transmitter, owner is 1" error is generated.
The debug output is updated every FixedUpdate and indicates that this desync remains until something else triggers another serialization (eg, hitting the sync button)