I don't have anything to add here,
@mvvRAZ and
@ECKHUAAA have provided a very precise and correct assessment of the problem.
@Juppiter have you checked the specification of N8ii when you run into a problem? The rated output of N8ii will increase from 760mW per channel to 1200mW per channel when you switch from P mode to P+ mode in 4.4mm Balanced phone out (or 420mW to 720mW with 3.5mm single-ended). If it sounds good in P mode but you heard abnormal noise with P+mode, I think we all come to the same explanation that you have over-driven your IEM already. The Infinity MK2 is an all-BA design, the distortion is more audible than DD based IEM when over-driven.
I think I need to stress one point: The different N8ii modes and their combinations are meant to make the DAP more versatile, but please don't assume every mode/combination will sound good for every IEM, so if you find some combinations didn't work out with our gears, don't panic, please stay with the settings that work fine. For instance, the P+ mode is clearly designed to handle difficult loads, and when you use it with high sensitivity and low impedance IEM, it can be inappropriate, and with certain types of music, it will even cause distortion.