Make sure the weep holes / drains on the outside of the window frame are clear of debris.
I’ve fixed one window twice. 1st Time the drain holes weren’t big enough to handle the mass amounts of water that flow down the side of the slide so since I never open the window I caulked it shut. The next time the same window 2 years later had trim shrinkage at the top of the bottom window so there again I caulker a 1/4 inch spot to fix. No more leak.
I’ve not seen felt in a window like that. Usually the lower channel is empty. It makes me wonder if the felt itself is causing water to wick through the weep holes.
Rain water should stay on the outside of the window. If it is leaking get it fixed. The water will get in the wall and mold will grow.