"Pancakes are on the house... No! Not literally!"
In maps where there are buildings(I can't remember if the same happens to caves), there's bound to be characters standing on/in the roof after the map finishes loading as you're arriving in the map.
When another character jumps into the air when standing beneath a low ceiling, they may glitch on/in the roof, so you see them on/in the roof, when on their screen their paws are on the floor. The same can happen after you arrive at a map, and others may even see you on/in the ceiling if the portal is below the ceiling. Correct me if I'm wrong.
So how do you see them standing on the ground again? The other character moves out of the building/cave and back then they'll be back on the ground, albeit temporarily if you have to re-log, but there's a simpler way.
Go to F3 mode by pressing the "F3" key on the keyboard, then press the plus(+) symbol a few times to increase the F3 speed, then press W, S, A or D and move the view away until the nametag isn't visible, then move the view closer until the nametags are back and voila! The other character will be standing on the ground(If the other person sees their character on the ground). If they glitch onto the roof again, repeat!