Upon powering a piston which is facing an unpushable block such as obsidian, and then breaking said block, the piston head will not render until it is fully extended.
I also managed to capture a screenshot with only the outline of the piston head rendering; I assume this was the moment that block 36 was replaced by the full piston head block.
Was able to reproduce 1.20.2
For clarity, steps to reproduce:
Place an unpushable block (e.g obsidian)
Place a piston facing into unpushable block and power the piston.
Break the unpushable block.
Upon breaking the unpushable block, the piston will extend. However in the frames before the piston is fully extended and in the process of extending, no piston head renders.