Whenever 2 villagers stand in trapdoors, they won't breed at all but they will breed if the trapdoors are taken out

To replicate:

Place 2 blocks next to eachother. Surround the floor blocks with blocks (2 blocks high) to hold the villagers. Place a bottom half trapdoor on the head level surrounding blocks and open them. Summon or spawn villagers. Close trapdoors, add many beds and give the villagers a few stacks of crops.

Expected result:
The villagers will pick up the food but won't "share" with eachother or breed.