A major bug causing automounting removable devices in Plasma to not work properly was just fixed upstream today:
The first half of the fix wound up in Plasma 5.45, and the other half in KDE Frameworks 5.46 (t couldn't go into 5.45 because it depends on the Plasma patch, and the next version of Plasma is being released after 5.45 is scheduled to land).
It would be really bad to ship Bionic with this bug, so since we're not planning to ship with Frameworks 5.46, we should backport that part of the fix: https://cgit.kde.org/solid.git/commit/?id=122a6cd8989a4bd3096fddea908a1c2b223be62a
If we end up shipping with 5.12.4 instead of 5.12.5, then we will additionally need to backport the other half of the fix: https://cgit.kde.org/plasma-desktop.git/commit/?id=8dedbcdf4197eccd5e06c18a66a63d15615c8171 into plasma-desktop