Is it intentional that setting the address time[51] via tag isn't possible.
I'm aware that the lease time is set in the range, but maybe the setting possibility for address time[51] should not be there under tags if it's only possible to set via lease time.
also maybe a misconfiguration on my side
We just collect all options that are valid via IANA and don't filter much. We don't know if somebody uses the option you have issues with successfully.
https://github.com/opnsense/core/blob/master/src/opnsense/scripts/dnsmasq/dnsmasq_dhcp_options.py#L36
The options directly supported by Dnsmasq will appear under "common" (means dnsmasq itself advertises these as being supported)
https://github.com/opnsense/core/blob/5b3b163ce0d93d41fcb141807c6d76389178ad1b/src/opnsense/scripts/dnsmasq/dnsmasq_dhcp_options.py#L78-L79