When prepping for painting the stairwell in my 1924 house, two cracks were fixed using fiberglass self-adhesive tape and joint compound. The compound was left to dry between applications and before painting. It's been about a month since the painting was completed and I'm now seeing that the patched areas are darker than the wall. The patches are under a window on an exterior wall. Any thoughts as to why this has happened and how I can fix? Photo attached.