I have a coloration issue where I applied filler. I can't make sense of it as it is sporadic and splotchy.

The floors are 1922 white oak. Drum sanded, then square buffed sanded 40,60, 80, 100. The full trough filler was applied, dried, sanded 80, 100. Bona woodline oil modified poly was used and i saw the light coloration appear.

What is this issue and how to correct it???!?

There could be different causes for light filler. A picture would be helpful. One reason would be that all of the filler has not been sanded from the surface which causes a hazy look. Another problem would be that the filler just doesn't have the right color to begin with to match this particular flooring. Gaps between rows that are light would be from the filler not being the right color match. Typically the filler needs to be as dark as almost all the boards because the wood will develop a darker patina over time. I have added universal tinting color to darken wood patch. Heart white oak can be very dark. The best grade of flooring is predominantly heart wood.