Thank you for making this post! Some users on forums I help manage said the logo changed for their homescreen shortcuts recently, and this appears to be why 
Maskable was super useful!
It took me a few minutes of comparing image URLs to be sure which image in Discourse’s settings I needed to change-- as far as I can tell, it’s the “manifest icon”. Which now that I type it out makes sense, but I was scanning the settings for something mentioning Android. Hope this saves someone else a few minutes