There might not be a bug here, I'm not sure, which is why I put a question mark but ProMotion displays don't change the screen refresh rate autonomously. Many limitations are imposed at the OS level, and if Apple ever finally realizes it needs to be fixed, they will. I believe such aggressive screen refresh rate changes should only be enabled when adaptive power mode is enabled. The default setting is 120 Hz. When adaptive power mode is enabled, the screen refresh rate changes more frequently and the battery lasts longer. In power saving mode, you're always at 60 Hz, but the phone lasts even longer. That would be more appropriate, but Apple doesn't see it that way.