Pistons and Slabs

I was testing to see if a bug someone mentioned could be replicated (MC-182452). I sadly couldn't replicate their bug but whilst testing, I found another. The pistons on the bottom activate when they're not supposed to if placed in the way the second (now last) screenshot shows, but then work as intended with the first (now second til last) screenshot.

UPDATE: Added two screenshots. Seems to be random..? (or dependent on the coords of the block)