Every time I connect my Bose QC35 headphones to my iPhone, the volume is reset to 50% instead of remembering what it was last set to. This occurs because I have Reduce Loud Sounds turned on (under Settings -> Sounds & Haptic -> Headphone Safety). When Reduce Loud Sounds is turned on, the volume resets to 50% every time the headphones connect. With Reduce Loud Sounds turned off, the volume remains at whatever value it was last set to.
Unfortunately there’s no logical explanation for this irritating behaviour, and knowing Apple this probably won’t ever be fixed. Reduce Loud Sounds is a solution in search of a problem. Volume Limit was far simpler, worked better, and didn’t have annoying quirks like this.