Great! You found it.
I think a careful painting job using matte black paint under the negative stage base and maybe even inside top of lens stage would be a good move to reduce the internal reflections in the area between the negative and lens.
Some "flare" is expected. You should be able to get good Dmax without the modification. But I still recommend the paint job. Also if you were using a condenser enlarger before, this is a diffusion enlarger which requires "harder" negatives to get the same results as before.
Your 50mm lens doesn't show it up because it's angle of view doesn't cover the entire stage.