"l2tp+" was/is a valid interface name for l2tp connections. It makes l2tp0/l2tp1 all match to the zone.
This is a MAJOR issue because it has left me without a functional firewall for over a month.
The error that is displayed isn't useful at all, but manually attempting to apply it results in this error:
/run/nftables_zone.conf:45:51-51: Error: syntax error, unexpected +, expecting comma or '}' iifname { eth1.2,eth1.10,eth1.50,eth1,l2tp+,wg0,tun0,vtun0,eth1.146,wg3,eth1.510,eth1.509,eth1.4,tun2,eth1.508,eth1.505,eth1.504,eth1.501,eth1.42,eth1.3,eth1.8,eth1.9,eth1.11,eth1.20,eth1.21,eth1.22,eth1.51,br142 } counter jump LAN-WAN