I think this might be an admin UI bug?
Platform version
2.3.9.141
Hardware version
C-7
see this. You have to manually specify a useable subnet outside of /24 then you can go back and set that...
Looks like they added support for the /23 to the UI since that 2021 post though, but the rule to make sure you don't do something really stupid seems to be tied to a /24 subnet still.
So yes, looks like a bug, you can use the endpoint in that other post to force it whatever you want (even if invalid).
Personally, I would just set the IP on the router DHCP and leave the hub on DHCP.
If you are not sure how to build the query string, I think this should work
http://HUB.IP/hub/advanced/switchToStaticIp?address=10.0.1.131&netmask=255.255.254.0&gateway=10.0.0.1&nameserver=8.8.4.4%2C8.8.8.8