RF100-500 Purple Dot Dead Center

"But on two lenses? Exactly the same?"

Probably every optical coating you have ever seen has been absolutely clear. But sometimes there will be a manufacturing defect that a coating has a slight haze to it. If you shine a bright flashlight on the coating, your eye can see a glow where light is scattering off. Generally, we think a surface can do four things with light: reflect, scatter, transmit, or absorb. Coatings should never scatter -- it is a defect if they do.

Then, like in my previous post described, some combinations of reflections off lens surfaces creates a hot spot on the sensor from the scattered light.

There are lots of possible causes of haze, for example, an air leak into the vacuum chamber during coating, contaminates in the coating material, or cleaning solvent left on a tray holding the optics during coating, incorrect temperature during coating, improper storage between successive coatings letting water vapor get one of the coating layers. (I suffered through a product recall for the last one, but I learned a lot from it.)

I have seen slightly hazy coating many times on various optics I have had made. But I have low volume stuff made for things that are much simpler than camera lenses. For me, typically a little haze does not matter. I think coating haze is extremely rare for lenses made by companies like Canon and Nikon. But it is kind of what this sounds like to me. If haze like that were occurring, it could happen in a great many lenses before it got noticed and fixed. From what you describe with it happening on two lenses, I think haze is more likely than dirt in the lens. The offending coating could be on one of the lens elements in the lens that you can't get to.

Admittingly this is a lot of guesswork on my part. It is just what it sounds like to me. I have seen quite a few hazy coatings over my career.

I am assuming you are not using a filter on your lens.   I would also be suspicious of any filters.