So in my case, I had the below rule in /etc/config/firewall to forward both tcp and udp 60002 to the internal host
config redirect
option target 'DNAT'
option src 'wan'
option dest 'lan'
option proto 'tcp udp'
option dest_ip '192.168.1.2'
option dest_port '60002'
option src_dport '60002'
fw4 translated it to the below, it got the tcp rule right, but the udp rule is missing a port number so all udp traffic was forwarded to the internal host. I've worked around the issue by having separate rules for tcp and udp and as a bonus, nft makes a little more sense than it did this time yesterday 
meta nfproto ipv4 tcp dport 60002 counter packets 9 bytes 476 dnat ip to 192.168.1.2:60002
meta nfproto ipv4 meta l4proto udp counter packets 12 bytes 1538 dnat ip to 192.168.1.2