Ipset is created without counters even if counters setting is enabled in Luci/Uci.

Create an ipset in Luci or Uci with counters enabled (Luci) or counters='1' (UCI). Save and apply. Reboot the router. Created ipset has no counters, e.g. nft list set inet fw4 myset returns

table inet fw4 {
	set myset {
		type ipv4_addr
		flags interval
		auto-merge
	}
}

but should return

table inet fw4 {
	set myset {
		type ipv4_addr
		flags interval
		counter
		auto-merge
	}
}

As I see there is no code that handles counters for ipsets in /usr/share/firewall4/templates/ruleset.uc, e.g.

{%   if (set.counters): %}
                counter
{%   endif %}

Start in /usr/share/firewall4/templates/ruleset.uc and trace how ipset definitions are rendered when counters are enabled through LuCI or UCI. Reproduce the configuration, reboot, and run nft list set inet fw4 myset; done means the generated set includes counter when the counters setting is enabled.