Fence does not hold chickens

The issue is when I built a chicken coop, using hoppers covered by white carpet, deep slate cobblestone and/or dark oak logs and fences, either spruce or oak. The chickens were walking on the carpet, and would migrate close to the fence placed on top of the deep slate cobblestone, and/or dark oak logs. When the chicken grew from baby to adult, the mob would push through the fence. When watching a second time, adults could also keep pushing against the fence and eventually force through the fence.

What I expected to happen was...:
I expected the fence to keep the mob in the enclosed area.

What actually happened was...:
The mob escaped the fence

Steps to Reproduce:
1. Build a base of hoppers, with white carpet on top of them.
2. Make a frame around these with Deepslate Cobblestone or Dark Oak Logs
3. Place spruce or oak fence on frame, all the way around
4. Stack it two high
5. Place Deepslate blocks or Dark Oak Logs on top and fill in the top, leaving one block open
6. Hatch 10-12 chickens from eggs on the carpet
7. Fill last block in, jump down and watch (It took less than one in game day for them to start on one, the second one I built it took almost 3 in game days for the issue to appear.