Hi.
I had this problem in 2.5.1 and 2.5.2, i tried ANYTHING, even opnsense but i had similar issues;
i found a permanent fix by removing all static routes and migrating them to firewall rules.
Let's say we have three connection WAN1, WAN2 and WANX, we want to use WAN1 and WAN2 in load balancing and WANX ONLY for some routes (no internet is available here!):
1- remove all static routes, they will be ignored anyway and may give error on UI
2- Configure your gateways and create a gateway group for WAN1 and WAN2, in this example we call it LoadBalance
3- Create like usual a firewall rule in LAN to redirect internet traffic to LoadBalance gateway
4- Create one or multiple firewall rules under LAN with source LAN NET and with destination the nework of your static rules
5- Put the "static route" rules on TOP and the load balancing rule on the bottom, save and apply
Everything should now work fine
Example: (note: networks in routes are redacted for privacy)
pfsense.jpg