Interesting. I may be overthinking here but I had a similar experience. WTBs are a scooped design in side profile and Arione is flat. I've used WTB Rocket Vs and Volts on my MTBs for maybe 8+ years, on gravel bikes for 3 or 4. Recent lower back tension meant some experimentation (as well as added stretching and adjusting myself as well as the bike). On road / gravel I've found a flatter side profile lets me shift fore-aft more and also lets my hips sit at a more natural angle, no muscles used resisting tilt or roll based on the saddle surface angle. I swapped my WTB for a Fabric Scoop, the flat one. Though the saddle hasn't been a total cure for lower back pain it did reduce it dramatically. The other thing that helped was a size up or +10mm stem for a flatter back and more space when climbing out of the saddle, just subtly less cramped.
Hard to isolate cause/effect but it seems that saddle shape affects hip angle or may be related to lower back tension.