When Vsync is disabled (enable_vsync=0 in pe_options.txt), textures randomly break across various blocks. The issue is difficult to describe precisely because it manifests in many different ways, but it generally looks like random texture atlases are being incorrectly applied to blocks.
For example:
Water may flash green or blue.
Command blocks sometimes show mismatched or incorrectly colored faces.
The affected blocks change every time the game is launched. This issue occurs regardless of the selected graphics mode—whether simple, fancy, vibrant visuals, or RTX.
Without any texture pack applied it still happens.
There was a previous bug report mentioning that this issue is related to Vsync, but I couldn’t find it again, so I’m submitting a new one. This bug has been present for several months.
This issue is being tracked at MCPE-202009.