As of 1.5-131 and later pistons no longer push items vertically with the same force they did before.
This can be seen from this video in 1.5-130: https://youtu.be/uTxb0ZuIVz8?t=355 (5:55 if the time stamp doesn't work)
That's from 1.5-134: https://youtu.be/W5L2er6oG2E

Both were recorded with the default push strength.

Log: https://pastebin.com/raw/89itRPAn
Mods installed: Quark + RealBench