Recently updated 1.20.1. I can place the planter box on the glass block itself, but I can't add flowers. I've tried several types of flowers and none of them will place. Shift + click also nothing. Not sure if this matters, but I also noticed there is no placing animation when trying to add flowers to the window box. All boxes that I've already filled with several kinds of flowers (vanilla and modded) all still have flowers.
#Window Flower Box Not Working
fierce beacon
gusty hare
I believe it's not a bug, the functionality of the flower box has been recently changed. It now only works with tall flowers.
If you want small flowers to work with the flower box again, try going to the configs and turn off "Simple Mode" for the flower box. That worked for me 🙂