Bathroom sink occassionally backs up into tub
A partial blockage allows some draining, but will backup when the water flows fast. It's not unusual.

You can clean the line with a snake - let the water run the whole time (to wash the dirt away, and keep down the smell), and go back and forth with the snake letting it spin and clean the walls of the pipe.

I also don't see a vent, but it may be hidden.