Why is the sound too low on the Xiaomi Buds 5 Pro?

It is due to the absolute volume on the phone. Absolute volume refers to the function of synchronizing the volume control between the earbuds and mobile phone or other audio device. By enabling the absolute volume function, the volume adjustment between the device (such as a mobile phone) and the earbuds will be automatically synchronized, and you can directly control the volume of the earbuds through the volume adjustment on the device, without having to adjust the volume on the earbuds and the device separately.
Please try the following steps to see if it resolves:
For Xiaomi/Redmi phones: Open the [Settings] >> [Bluetooth] >> tap the [>] icon beside the Xiaomi Buds 5 Pro >> turn off [Absolute volume].