It's a paper coating issue. I've had it happen with Fotokemika Emaks, Fotokemika Varycon, Foma 132, and Kentmere Kentona.
Lith has a tendency to reveal defects in the paper that simply doesn't show up with regular chemistry, for various reasons I am unfamiliar with. What's particularly difficult is that it isn't consistent from batch to batch, unless you use something like Ilford Warmtone, in which case it'll be consistently good without such artifacts.
Good luck in sorting it out!