[The_bluester (Paul)]

Depending on how the program you are using auto stretches, flats can look like they show extreme vignetting just because the range of data is typically very small and it will be stretched so it fills the histogram.

This is what my current flats look like (LRGB) but if you don't stretch them you would be hard pressed to see any vignetting at all. This is on my refractor, the ones off the newt look even worse as there is some vignetting from the secondary mirror so it is even more uneven looking.

Edited to add, this is from my ASI2600MM, 36mm unmounted Astronomik filters (Which are basically a mounted filter, they come in an aluminium ring to make putting them in a filter wheel easier) and my Stellarvue SVX80T at F4.8.