This happens because Bluetooth has two modes: one for listening to higher-quality audio, and another for both listening and speaking through the microphone. When Bluetooth switches to the second mode, the quality of the audio that you're playing is reduced until the microphone is no longer in use.
Follow this link for more info about this said:
https://en.m.wikipedia.org/wiki/List_of_Bluetooth_profiles
Source- Apple/Wikipedia.
Simple answer
It can't be helped because this issue isn't because of your earphone.
Its just that bluetooth codec have specific profiles when it operated and those change automatically upon usage.
Hope it help
Last edited: Nov 30, 2020