Odd behaviour when pistons retract slime-blocks and regular blocks

Pistons don't retract slime-blocks in some situations.
The first three screen-shots show the issue (and of course both pistons are sticky). You see that the regular block gets retracted but the slim-block doesn't.
If you just change the slime-block and the regular block, both get retracted.
So this isn't regular behaviour.
And this works also horizontally (last two screen-shots)