Okay, a slightly edited, less extensive set of related changes is available at More context applet improvements (!86) · Merge requests · Multimedia / Amarok · GitLab (the MR also includes some Photos applet fixes and uses lyrics.ovh for lyrics).
I still haven’t managed to reproduce this myself (or, not really reproduce: setting color: palette.window in QML file for those labels does yield a similar result, no idea why), and I couldn’t even reproduce the similar issue I had earlier with one applet any more. However, looking at code and various resources, I guess it might be some corner case behaviour difference in SystemPalette between QtControls1 and 2. Not quite sure how and why, but maybe adding an explicit binding to the correct Kirigami colour helps.
Many of the context applets would be straightforward to port from Controls 1 to 2, but apparently there might be surprises, so I guess that’s something to do later during Qt6/KF6 port, then.