Please tell us some basic information before asking for help:
Model Name: Asus ROG Phone 5
Firmware Version: 2106.86
Rooted or not: Stock
Frequency of Occurrence: Every time
APP Name & APP Version (If your issue relates to the app): All media playback apps
Using the 3.5mm jack into a high impedance load (external amplifier), the auto-limiter kicks in at 80% volume position. This causes the music to peak and dip as the song progresses, and is very annoying.
There is no issue at top volume position, and no issue below that specific position. This is where the slider would hit the yellow warning zone on all other volume sliders (but the media volume slider, understandably, does not have the yellow zone).
Cannot test with low impedance load as the volume would be at level of hearing damage.