Description of problem:
$ firewall-cmd --permanent --new-ipset=temp4 --type=hash:ip --option=maxelem=65536 --option=family=inet --option=hashsize=4096 --option=timeout=14400

is not passed to the backend: No timeout parameter available in ruleset.
This makes nft backend unusable if you want timeout sets and you are stuck with iptables.

$ nft list set ip firewalld temp4

table ip firewalld {
        set temp4 {
                type ipv4_addr
                flags interval
        }
}


Version-Release number of selected component (if applicable):
Name        : firewalld
Version     : 0.9.3
Release     : 7.el8
Architecture: noarch

How reproducible:
try the commands above.

Additional info:
Maybe related to units, as the native nft command requires "14400s" to add an set and firewalld works on an integer implicit assuming seconds.