Actually not as strange as it may seem. See this post for a high level description of how Mosaic discovery works. There is a whole family of different protocols running on your network and just because HTTP/TCP (as used by a browser) is working that doesn’t prove that mDNS/UDP (as used by Mosaic for discovery) is.
So unfortunately the notion that a network is an atomic entity that is either completely working or completely broken is an incorrect oversimplification. Networking equipment is not all created equal and some products don’t handle mDNS correctly. I am not saying that is the cause in your case, but it is one of the more common causes of discovery problems reported here on the forum. Hopefully Richcom can help you get to the bottom of it.