Thank you for asking this question in the support community. The order of operation for this application is as follows:
Incoming Packet ----> PBR - Route Lookup ----> Access Policy Allow/Discard or NAT Source
So, for your configuration the traffic is being sent to the next hop of 188.8.131.52 with the source IP address of VLAN 100.
Example 5. on page 33 describes a similar configuration in the Configuring the Firewall (IPv4) in AOS document.
I hope that makes sense, but please do not hesitate to reply to this post with any additional questions or information. I will be happy to help in any way I can.