I thought it was better to create this post than bump the old one (Group uploading decal bug).
When you are uploading a decal to a group from the group’s develop page, the decal list part of the page will refresh to your (not the group’s) decals.
You can still create a decal for the group in this mode, and your URL stays on the group, only the decal list part changes.
GIF example (provided by @GeorgeOfAIITrades):
I do not know exactly when this started occurring, but it’s been happening for me as long as I have been uploading decals to a group. The first bug report for this was posted in 2015.