I ‘reproduced’ the bug, possibly. Possibly it never was quite what I thought it was.
I got hit by it again - couldn't save the contents of the form to set an ipv4 static ip and the save option was grayed out. Why ? Bug !
Well.
I realised that the netmask box has a fake ‘value’ showing in it which is actually possibly non-existent. There is some text 255.255.0.0 showing which, might be in lighter gray, I can't be sure about the color. But it seems that actually the field is ‘invalid’ in reality unless the user types some text into it. The text 255.255.0.0 is actually not a useful initial value! (It's just useless decoration to remind the user.)
This is silly. It should be empty initially and if so then needs to be clearly visibly so. What is really silly is that this is all the more confusing because in my case 255.250.0.0 happened to be a plausible default value as it is the correct size for 192.168.0.x which is what I happened to have entered.
So problem solved. Whether or not that was the story in the old case, I do not know. Perhaps I always got misled by this shockingly weak UI design.