Root cause:

RTS table is created after setting routing rule of the pcb, so response packets from this request(pcb) can’t be routed back successfully. For later requests, because RTS table has already been created, response packets can be routed through RTS table.
Solution:

Move rts_create() before setting pcb’s routing rule (in clickudp_open()).

client runs 3 times dig @8.8.8.8 google.com, which is DNS request, and make sure all requests can recieve its response.