I have a piston platform that travels up and down on a 4 clock (2 redstone ticks on, two redstone ticks off). Solid blocks (grass) on top of the sticky pistons and flowers growing on the grass, and a ceiling directly above where the flowers grow.

What happened in 1.14: flowers would break when the piston platform extended, the flower items would push up against the ceiling and drop down again, staying on top of the grass blocks on the piston platform. With a 4 tick pulse, the flowers would phase upwards through the ceiling to be collected.

What happens in 1.16: As the piston platform extends, flowers break, phase downwards and completely through the grass block and get trapped inside the pistons. When the platform is stopped, they rise back up and sit on the grass blocks again. However, flowers close to the edge of the platform will phase through a 1 thick wall around the edge of the platform and be ejected to the ground.

What I expect to happen: I expected behaviour as per 1.14. Items phasing down into a piston are unmanageable, so this can't be desirable behaviour. There was no indication in any release notes that this functionality would be changing.

I've attached a video of 1.14 and 1.16 behaviour and some additional screen shots from 1.16