Block snap skipping one

I was creating my character, and suddenly i’ve discovered a weird bug, which turns a certain side snap totally asymmetric when comparing to another just one block aside.

According to game sounds, you can hear the number of times each block was snapping, and an explicit difference can be seen between these two:

It only happened inside this grid area, on character’s left leg of the left side only, and oddly it didn’t happened with another character parts.