Pinging from the next hop (the ISP side) is completely different from/to the internet (more than 2 hops away). If it's the next hop, it's a "connected" route so the FGT always has a route.
If you change the source IP of your pinging to have the second wan interface IP toward the internet, it still needs to follow a proper default route unless you have a specific route like 184.108.40.206/32 to the second wan GW.
Since you're using the second interface for internet already, there must be a default route pointing to the direction already. The question is "Is it always the primary default route like with higher priority (lower number)?"
It's up to how you implemented two default routes to split/load-balance traffic toward the internet.