Hi Tom. I am more inclined to think the problem is not the paper or developer, but weak/tired fixer.
I have a few old papers (as in expiration date c1950 old) that are so horribly fogged that in dektol they go black in seconds, yet process in weak lith with no fogging. Perhaps if your lith developer is on the stronger side, then if the paper is fogged you will see it.
Big fat white borders chew up the fixer pretty quickly, especially so with fiber paper in a single bath process.
If you don't think the fixer is the problem, then test your paper in some regular developer for fogging.
Also, I think this paper has a note on the data sheet concerning safe light precautions. With Foma MG I usually turn all the lights in the darkroom off for the first ten minutes or so, because my regular orange lights will fog the paper if it's in the tray for extended periods (15+ minutes).
Could be any of these things, or combinations thereof.