Cat Peeing On Litter Catching Mat

I got a kitten a few weeks ago, he is about 3 months now. When he uses his own litter box he is totally fine, everything normal.

On my older cat's litter box, there is a litter catching mat, to keep litter from being tracked everywhere. My older cat has never had a problem with this before. However, when the kitten uses the older cat's litter box, he goes on the mat instead of in the box. Today I put him in the box a few times and observed him, and he kept climbing out of the box and squatting over the mat instead. I managed to catch him in time and put him in the box while he peed this time, but he still climbed on to the mat to scratch and try and "cover".

I don't understand why he thinks the mat is where he should go, as the litter box and type of litter are the exact same as the ones that are in his room. Has anyone else experienced this before? Any tips for how to break this habit before it becomes ingrained?

We have had cats occasionally do this - often when the mat has a decent amount of litter on it. We tried to keep it cleaner but they are hard to wash Olive still got confused. We ended up getting a new one and that seems to work fine. We just try and keep an eye on it.

With a kitten, maybe remove the mat for a while until he is being consistent in that box and then add it back later once he has the hang of it?

As KarenKat suggested, it might be that there is enough litter on the mat to attract the kitten to use it instead of the box. However, the kitten might actually dislike something about the litter or litterbox, but goes in it anyway when confined to his own room, as he has no other choice.
When he has the chance to use the other litterbox with the mat then he will opt for the mat instead. If the litter boxes are high sided, try getting the kitten a lower sided box and perhaps set it aside the older cat's regular box - just to see if the kitten will use the lower sided box rather than the mat.
It is also possible that the kitten doesn't want to go in a litterbox that has been used by another cat, so when he is in that area and has to go, he chooses the mat.