Originally posted by jonnin:

its from a lack of support and it just takes a while for the game to compute that its not good enough. Adding support below it is the only fix. Often you just need wood iron or iron embedded in the stone to shore it up, invisible and easy to do.

Incorrect. It has support. This is the kind of comment that I designated as try to avoid. Thanks anyway, amigo. I can sit and watch this same block for an eternity, to no change or effect.

However, moving away from it, to the point it despawns, then back towards it? Or - logging out and back in? Sometimes, but not every time, causes the popcorn.