Come to think of it, I did hear about this from someone a while ago who was using some sort of public WiFi HotSpot (bad idea in general) which had a broken redirect page and in turn didn't open up the firewall and gets you that 'partial' connection that causes all sorts of problems. Was on both macOS (10.12 I think) and Linux (some Ubuntu version) at that time.

Seems to be an interesting problem: if it works fine with no connection and fine with a complete connection but not 'in between' (which is the best way I can describe it so far) you'd think it must be some common library or component in a network stack that causes this. macOS has some reachability system that might be in play here, perhaps if it flags the network as 'reachable' but then gets REJECT'ed it goes bad? Or the other way around: marks network as 'unreachable' but traffic flows anyway?