I've informed ingbrzy about it and how to fix it. The next release should solve the headphones volume issues for Mi 5 and Mi 5s Plus.
This volume restriction was put by Xiaomi since MIUI 9 Beta 7.9.14, for unknown reasons, using an optional overlay file that decreases your device's headphones volume significantly.
For now you can fix your headphones volume by deleting "/system/etc/mixer_paths_overlay_dynamic.xml" and rebooting your device, using TWRP or, if you're rooted, any root browser.
The next Xiaomi.eu versions will not include this file.