...is it because the outside of a fast belt is going too fast for a player without exoskeleton to run against?

Just to be clear, if the entire belt is a fast belt this still happens. I can run against it just fine until I reach the corner, then I'm stuck.

I made a multiplayer game that demonstrates the behaviour. Turns out only the corner belt needs to be a fast belt.

To activate the bug, just run along the lowest row of iron ore miners while trying to go SE. Player will be stuck at the fast corner belt.

I noticed that I get caught immediately in ...

This seems to happen only under very specific circumstances; In multiplayer, if I run against the direction of a fast belt going between miners and I push against one row of miners, i.e. actually trying to go NE on a belt going W and the belt enters the row of miners from the south so that there's a ...