I’ve never really seen or dealt with an address space larger than 8 bits. And indeed 16 bits seem would be the next I’d think of, instead of starting in the middle of an octal, for very large networks where 254 addresses are not sufficient.
To me it seems like the router is passing the address space in an uncommon way, using the network mask behind tha backslash, instead of the CIDR bits notation, at least based on the output.
However, ifupdown seems to indeed handle this somehow strange. It throws an error message, but then says it bound to the address, while nothing was bound to.
I’ll test this when I find time. Generally I think any network size is supported, as long as the format is correct. While other network managers/OSes may handle things more graceful, at least Debian/ifupdown does the way your DHCP server provides the info.