This Week in Plasma explains how we deal with post-release issues in the wake of Plasma 6.3—and how more testers are needed, and lists the UI improvements arriving in 6.3.1, and the new features lined up for 6.4.0. and beyond.
https://blogs.kde.org/2025/02/15/this-week-in-plasma-post-release-polishing/
If the issue is KDE specific, the bugtracker is over at bugs.kde.org. Make sure to try and find out whether someone has already made a report.