New Pure Maps release 1.29.0 contains a patch that applies audio role notification to voice directions. Normal audio, e.g., music, is no more stopped, but damped now.
VW carkits have an infamous bug, that auto-mute Bluetooth audio after music was paused/stopped. When the next voice direction came in, unmuting took some time, cutting-off part of directions. This problem does not happen any more with the new Pure Maps release.
Using Ubuntu Touch as daily driver got a little more comfortable again. Thanks @rinigus and jonnius!