Beginner Gardening: Black spots on Spadix, 1 by GreenSh

GreenSh wrote:
Bought this (see attachment) peace Lilly from Lowe’s 3 months ago. Placed it in a South side window, Light level is low, pot has drain hole. I water once a week 250 ml with tap water. I repotted without disturbing the roots, used regular potting soils and perlite on top. 3 weeks back, I noticed white powder on leaves, then black spots on spadix and brown streaks on flower. I thought it’s just done blooming. But a week ago, I started noticing black spots on young spadix.
Posted my problem with detailed pics in many other forums. People suggested many things but no one knows exactly what’s the problem.
Today, I loosened the roots a little bit and repotted with new soil. Didn’t find any rot/dry roots, no mold or mites on leaves. Then I watered it with Miracle plant food (as directed). I also pruned the infected stems all the way down.
If anyone knows what it is please help me.