In the past as I started structured training most of my biggest FTP gains came after SSB1. My first year on the trainer I moved up from 232 to 261 after SSB1 and then from 261 to 270 after SSB2. Both mid volume.
As I’ve gone through the progression a few more times and my base over the years grows, the jump isn’t nearly as much. Each time after SSB2 though, I’ve certainly noticed improved fitness, it just hasn’t necessarily directly been an FTP gain.
Now, based on your test it seems like something could have been going on there, but I’d stick with your tested FTP for a bit perhaps and see how those workouts go. If things are too easy, nudge up manually after or just consider retesting if it would give you piece of mind and it fits into your plan.
Regardless, feel proud of that 40W total increase. That’s MASSIVE and a huge win.