I have experienced the same with an older lens (on a different camera though).
The way I understand is that a flat surface of an uncoated lens or filter acts as a partial mirror. Older lenses are particularly problematic. There is no problem with film cameras because the film surface is not glossy. Sensors are highly reflective and any light entering through the lens is bouncing back and forth between the sensor and the reflective surface. The effect is most pronounced at smaller apertures (f/16 or smaller) an with infra red filters (gives rise to a well known hot spot).
The lens I had problems with was a Tamron 90/2.5 macro. Beautiful manual lens, however it has a flat rear element and it is not properly coated (the lens was designed for film...). It is now gathering dust.
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