So, ever since the DevForum icon was updated, desktop notifications display the icon of the browser rather than the DevForum:
Chrome:
Edge:
I’m guessing these are just placeholder icons because the DevForum may not be providing them.
However, this does not happen on mobile because push notifications consist of the Discourse icon, in which the notification contents were unaffected by the DevForum icon change.
Not only the notifications, but the tab icon above the address bar doesn’t show the count either. I’m using Chrome, but it apparently happens no matter which browser you use.
What it did was show the number of new notifications instead of the standard icon (the standard forum/studio icon was faded and the number was shown on top of it, to be more precise). Unfortunately, I don’t have any screenshots of it, but started happening around the same time as your + OP’s problem.