After my workaround described above:
vyos@vyos# sudo netstat -tulpn | grep 993 tcp 0 0 192.0.2.1:993 0.0.0.0:* LISTEN 513 01/haproxy tcp6 0 0 2001:0db8::25:993 :::* LISTEN 784 61/conmon
and then when configuring the other way around:
edit] vyos@vyos# set load-balancing haproxy service wan-993 port 994 [edit] vyos@vyos# commit [edit] vyos@vyos# set load-balancing haproxy service wan-993 port 993 [edit] vyos@vyos# commit [ load-balancing haproxy ] TCP port "993" is used by another service [[load-balancing haproxy]] failed Commit failed [edit] vyos@vyos# sudo netstat -tulpn | grep 993 tcp6 0 0 2001:0db8::25:993 :::* LISTEN 784 61/conmon