Yes having the same GW for multiple WAN IP:s worked (at least for me) fine for a while. This is basically the only option you have if you want to run with multiple wan IP:s and your operator is providing you with multiple IP:s with DHCP (mine gives up to 5, no static IP:s available) . Off course for monitoring of GW one must use different targets for every GW.
For testing purposes I did do a fresh install of Pfsense 2.4.4-RELEASE-p2 and the problem seems to stay.
Annoying part is that this setup now works, for a while, then it goes offline, and soon works again :).