Hey! You’re right in thinking that below 320px, it’ll revert back to the desktop breakpoint.
You’re also right that the “px” of the device screen resolution doesn’t equal it’s px width for web measurements. (Most modern phones screen resolution width is larger than the “px” unit used to determine screen. In short, they’re not comparable values even though they use the same units).
With that said, it could be a combination of both the device, and browser used on each device.
Would you be able to open a ticket for this? If so, the best place to report an issue is via the steps outlined in this topic.