I do not think that is the case, yesterday I did some more testing, I did se cobblestone blocks. I did also was on debug mode and enabled the stability check, color of weakest point was kind of light orange and then adding more blocks on orange color it just colapse.
Also, this same build I was made on alpha 19 and 20 and it was stable, if there was a huge update on stability on alpha 21 then could be possible.