I think this is a bug, but perhaps its not.
But it seems some things like circuits have a very high minimum payload
Most items are just the default stack size, but for me circuits I think were 2000

This just started happening for me as well.
It just keeps filling up my platform + cargo bay.

Just trying to get some stuff to go to another planet and it keeps filling up

I think this issue should only come up if 1 or more of your stations has a train limit set higher than 1.

I wasn't clear from your post if you had a single train and only stations with limit 1, but going to assume you have multiple trains, and possibly 1 or more station with a limit higher than 1 ...