I have a prototype bamboo farm in a creative testing world that worked fine last snapshot. When I turned it on this snapshot, the bamboo items started going through the glass surrounding the bamboo. I replaced the glass with stone and items still went through the blocks.

What I expected to happen was for the blocks to stop the items and contain them.

What actually happened was the items going through the blocks.

Steps to Reproduce:
1. Place a horizontal piston 4 blocks above the ground.
2. Place blocks surrounding where the piston head would be if it was extended except above.
3. Throw items in front of the still unextended piston head and place a block above them.
4. Power the piston. In 1.13.2 and 19w04b, the items will stay in the contained area. In 19w05a, the items will be pushed out and will land on the ground. If done at ground level in 19w05a, the items can be pushed down, going through blocks until it reaches an opening - void, cave, abandoned mineshaft, etc.

 One picture is of my bug testing set-up. The other two are of my farm (the command blocks are for keeping the dispensers filled with bonemeal).

Edit: I did some additional testing, and I think it is an issue with the items themselves not interacting properly with blocks. Replace the piston with a normal block, and place another block inside the items that are thrown out. Previously, the items would be pushed above the block placed in them. Now the items will be pushed downwards like with the piston method at ground level. I changed the bug title to reflect this.

Edit 2: As of 19w08a, this has been partially fixed: items will properly pop up when there are blocks surrounding them and another block is placed inside them if another block isn't above the items (items properly respond to the the steps to reproduce in the first edit). If items are in a block with other blocks on all six faces, the items still glitch through blocks improperly. This time they will either go to one of the sides (x or z axis), or they will go down. From my testing, it seems that it is about 50/50 whether the items go to the side or go down.