Buffer Underruns occur when there is more data coming into the audio buffer than can be sent out, there are many many many reasons this can happen. I can’t speak to your specific scenario, but I have been working with digital audio for about 20 years. A buffer underrun is a very common error working with digital audio.
All I can say is that your custom install was not setup the same way as the official image, all else is guesswork.
^^ my other hobby