Description of problem:
During testing of pulseeffects package I hit an issue in plasma5 which I reported to PE upstream,
https://github.com/wwmm/pulseeffects/issues/99#issuecomment-346187296
and then PA upstream on irc. After lengthy debugging I quote the PA dev:
"
This is a bug in PulseAudio, in module-device-manager. The reason why this doesn't happen on non-KDE systems is that module-device-manager is only loaded when using KDE (see /usr/bin/start-pulseaudio-x11).

When an application requests a specific device when creating a stream, module-device-manager will let the application use that device initially, but it doesn't remember that the stream had "special" routing that shouldn't be changed. There are many situations where module-device-manager will check the stream routing and move streams if their routing doesn't match what module-device-manager thinks the routing should be.

An extra bit of information should be added to the internal stream state: "this stream is routed according to the application's request". When that bit is set, module-device-manager should refrain from moving the stream. If the stream is moved for any reason (e.g. requested by user), then that bit should be cleared, since the initial routing isn't relevant any more.
"

Version-Release number of selected component (if applicable):
Tested in 10.x and 11.1 (from Mga svn patched with extra debugging code provided by PA upstream)

How reproducible:
Always

Steps to Reproduce:
See linked pulseeffects bug report