SUMMARY
Extra space inside "100%", making it "1 00%".

STEPS TO REPRODUCE
1. Open Plasma volume widget's popup.
2. Set device or application volume to 99%.
3. Drag the slider to the right a little to set the volume to 100%.

OBSERVED RESULT
The text to the right of the slider becomes "100%" with a little space between "1" and "00%". I estimate that the "1" is misplaced one pixel to the left from its normal position which puts it a little far away from "00%".

EXPECTED RESULT
Should be normal text "100%".

SOFTWARE/OS VERSIONS
Operating System: Fedora 30
KDE Plasma Version: 5.15.5
KDE Frameworks Version: 5.59.0
Qt Version: 5.12.4
Kernel Version: 5.1.16-300.fc30.x86_64
OS Type: 64-bit
Processors: 2 × Intel® Celeron® CPU B800 @ 1.50GHz
Memory: 3,8 ГиБ

ADDITIONAL INFORMATION
"100%" looks good if the slider was moved down from a value higher than 100%, e.g. if you firstly set volume to 101% and then move the slider to the left to 100%.