nmap port scanner shows open ports when destination is to a VS on the front end

1.Check the distination ports of your virtual servers as they can be listening to all ports:

https://support.f5.com/csp/article/K6018

2.Also if the F5 vip is with "Loose Initiation and Loose Close " this means that any client packets is accepted without 3 way handshake

https://support.f5.com/csp/article/K13558

3.you mention RST to see if the F5 is returning the RST enable special logging https://support.f5.com/csp/article/K13223 and you may do nnnp tcpdump https://support.f5.com/csp/article/K13637. Also check if nmap is not triggering syn cookie protection https://support.f5.com/csp/article/K74451051#syn-cookie-status