20w49a, 20w51a, 21w03a, 21w05b, 21w11a, 21w15a, 21w16a, 21w19a
This is caused by the fix for
MC-108469: entities are saved separately from the chunks they are in. If a chunk has a scheduled tick that will fire the moment it is loaded it will cause blocks that check for entities (detector rails, pressure plates, string, etc.) to determine that there is no entity present, so they turn off.
Expected behavior: The blocks stay turned on because they are activated by the corresponding entity.
Actual behavior: Depending on the timing of the chunk reload some blocks turn off and back on a moment later.
To reproduce in the attached world:
- Step on the pressure plate, this will clear the red glass blocks and teleport you away
- Step on the second pressure plate to turn back
- Observe that sometimes the timing is right and will cause some command blocks to place a red stained glass block above them.
A few tries might be required to observe the bug, depending on the exact timing of the chunk
Video showing the issue in the test world: https://youtu.be/hVQeCybGnU8