So after trying many different things, I have now determined this:
It seems there is an issue (maybe just with my particular rom bulid) whereby the "Google App Data" doesn't sync properly. I happened to add another google account to my device, and doing so must have triggered something. Even after deleting the second google account, I can now receive the Icam notifications...and some from other apps that I had no idea I was missing.
Again not sure what the true cause is, but it appears this is an operational workaround.