The most likely cause is stray light leaking into the stacked filter setup and bouncing between the two filter surfaces, creating flare that looks like a washed-out area. This is especially likely if the outer filter or holder doesn’t have good light sealing or if the filters lack strong anti-reflective coatings.

A wide lens can make filter-holder problems more visible, but your telephoto result suggests the issue is not only focal length—it may also be alignment, reflections, or even a slightly warped filter becoming more obvious when stacked.

What to try:

• Block side/top/bottom light more effectively with a darker, more opaque cover than paper.
• Check whether either filter is warped or not seated flat in the holder.
• Inspect the holder/filter alignment carefully.
• If possible, use a filter with a foam gasket or better sealing to reduce light leaks.
• Test each filter position/order to see whether one filter is the main source of reflections.

In short: this looks more like light leak/internal reflection from stacking square filters than a problem with the camera or lenses themselves.