I couldn't find a duplicate of this, so I assume it hasn't been reported yet? I found some about moving blocks constantly where they disappear, but not one where they flash in an out of existence, sometimes leaving ghost renders.

That's basically what happens. When a lot of blocks are pushed at once, even with the new rendering system in place, the rendering engine just fails to properly render all the changes in block positions and push animations. The provided video is of a 16x16 piston door that uses 32 flying machines, each with 3 parts to it. Everything is moving on the same tick as the rest of it.