Try to record a clip of that happening, see if it's audible on the clip, maybe it's hardware (as in Windows chooses a different device to playback Spotify while TruckersMP is using a different one, what I mean by that is in my case, on Sony WH-1000XM4 my game (not necessarily ETS) would play on Sony blah blah device, but some other things would playback on Sony blah blah (Phone) device or something like that).
Follow the steps provided in the screenshot to set your desired device as default, and if the issue still persist, try checking the spotify uses a device that you specify, not the default playback one.
Step 1. is right click.