By default, FortiOS automatically adds "static" routes to the destination of phase-2 selectors when the phase-1 is of type "dynamic".
Look for the "set add-route enable" command in phase-1.
FGT-01 (root) # config vpn ipsec phase1-interface
FGT-01 (phase1-interface) # edit TEST-VPN-P1
new entry 'TEST-VPN-P1' added
FGT-01 (TEST-VPN-P1) # set type dynamic
FGT-01 (TEST-VPN-P1) # show full | grep add
set add-route enable
FGT-01 (TEST-VPN-P1) #
Without this, or when using 0/0 in phase 2, you will need to use static routes.
If you want to see what routes are being injected into the static RIB by the IPsec engine, then use:
diag vpn ike routes