yeah i understand your situation
i am like that too, not knowing what is causing a problem just gets to me sometimes and i really hate doing work-a-round fixes without knowing what the real problem is.

it is strange though that with the oil level at normal levels that the oil is finding its way into the breather cap. We have covered all the possibilities that may cause that so it has to be something you can't see. And from the looks of that tank assembly its not very easy to see inside of it anyway, only 2 places to look, the breather cap hole or the fill cap nozzle according to the diagram i saw from JLG on the hydraulic tank.
It still comes to mind that somehow the oil level is getting high enough for it to enter that cap nozzle. Maybe the force from the returning fluid is making it swell inside where you cant see it, but that doesn't seem logical either because of where the supply/return line is but i have seen stranger things before.

honestly without being able to get in front of it and see what is going on i am just throwing out educated guesses anyway.
I wish you luck and hope you do find out why it is happening.
If you do please come back and post what you find, i am very curious myself to know :o)

have a great july 4th