In 1.21.8, confined mobs would randomly move around and quickly spread out to fill the space they’re in. If the space became larger, such as by opening a door, the mobs would fill the larger space. Any number of mobs would do this, not just when they are at or near entity cramming.
In 1.21.10, confined mobs do not move at all unless they perceive a path to “outside” or to the player.
I’ve built several mob farms with mob sorters and zombie villager curing chambers that depend on the 1.21.8 behavior. The attached screenshot shows zombies in one of my farms not going through an open door from a 1x1 room into a 1x2 room.
The same issue affects witches. I used to move witches down a hallway by letting them randomly walk through opened doors and closing the doors behind them. They no longer move at all unless they perceive a path out or to the player.
Environment
Java Edition; various flavors of Linux; local singleplayer game and vanilla server on LAN.
I installed 1.21.8 and reproduced the old behavior in a test world. Not only do the mobs immediately spread out, they continue to jostle each other as they randomly move around. In 1.21.10 they remain completely static.