Sometimes when using the PlayCanvas UI, dragging materials stops working. I can’t narrow down when it happens, but it happens quite frequently.
It happened to me earlier, and I was able to record the issue.
Dragging the material over a different UI element seems to fix this, hopefully that’ll help track down the bug / solution.
What I normally end up doing ( see 0:35 ) is dragging the material onto the viewport, then off it, and then back into it again.