1.5-stream-2025-Q2 moved from the legacy Vyatta WLB solution to the VyOS WLB solution. You can see that in this blog post: VyOS Stream 1.5-2025-Q2 is available for download. What you’re seeing is likely a bug in how the the WLB solution generates its SNAT rules. Can you submit a bug to https://vyos.dev/ detailing what you’re seeing?
With that said, since you’re already defining your own SNAT rules, you can maybe solve this by disabling SNAT for WLB.
set load-balancing wan disable-source-nat
You can read about that here: WAN load balancing — VyOS 1.5.x (circinus) documentation
That may cause unexpected results though since the WLB chains are run after the SNAT chains. I haven’t tested the WLB solution, so this is all from a quick glance at the source code.