Hello everyone,

I'm very new to woodworking and I'm enjoying it a lot.
I'm making some shadow box frames for some art and running into an issue when I'm staining the wood.
I'm using pine for the frames and maple for the molding.
After gluing (then waiting 24 hours), woodfilling (waiting another 24 hours) and sanding smooth, I then stain the wood with a foam brush.
I get a glossy discoloration around the area where I have applied woodfill.
What's weird is that sometimes it doesn't always happen to the actual cured woodfill itself, but to the surrounding area.
This happens even after I've spraypainted with clear coat or with white primer.
I've tried this on multiple pieces with one or multiple coats of stain.
Would be very grateful for any advice or insight.
Please let me know if you need more information.
Thank you.

I am using Elmer Wood Glue, Elmer Stainable Woodfill, and SamanN TEW-108-12 Odorless Black Wood Stain.
Rustoleum Clear Coat and White Primer spraypaints.
I am sanding by hand with 220 grit sandpaper.