UDP Virtual Server, VIP return 'port unreachable'

Because the UDP protocol does not have the same options like the TCP to use TCP FIN when a normal connection is closed or TCP RST for issues for example with F5 for when all pool members are down the F5 devices uses ICMP for UDP VIP and healrh monitors. Just do normal investigation in the /var/log/ltm if the pool and pool members are going down. Check if the UDP timeout in the UDP profile is causing this maybe if there is an irule and so on.

https://support.f5.com/csp/article/K6143