That's an odd one. If my Mac is offline it works just fine. Perhaps your filtering/firewalling isn't complete so it gets partial connections and then times out on the rest? If you do that in large quantities, any OS will start to show trouble.
Timing out packets instead of denying them could certainly be an issue (I run across this a lot with internet being down while the router and internal DNS is still up).
But your claim that “any OS will start to show trouble” is not how it’s supposed to work, nor how it used to work before 24/7 connections, nor even how it should work assuming you’re ok with phone-home daemons.
Of course it's not how it's supposed to work, but that is how it tends to work ;-)
An OS in general has a few layers with caches and monitors and resolvers etc, and if you block a few of them, in a partial manner, they tend to get in to an extreme version of the bus bunching problem. Windows still does that with their 'network identification' where sometimes something goes wrong in the probing process and the connection hangs on "identifying" indefinitely. And that's just a silly "optional" service (which should default to the public profile and only change from there instead of defaulting to no connection at all).
Flaky network stacks aren't rare. I've experienced plenty of consumer-facing operating systems that will freeze on boot for 10 to 60 seconds if the network interface is up but DHCP is unreachable.
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?
