I tested the recent version of KDE using Arch Linux for a week and it actually has several problems.
One of the problems is in the calendar and another in the panel that sometimes disappears or in an alternative management of tasks and among other problems. Due to these problems and aiming for stability, I decided to return to my dear Manjaro Stable using XFCE, with new update it’s all working very well.
Most of the biggest initial Plasma 5.13 bugs are fixed. To be honest, I don’t noticed any problems that you encountered. What is wrong with calendar? Have you filed bug reports?
The things is, people see a problem and dismiss it as DE fault instead improving it. I found Plasma 5.13 problems and submitted them and because of that they are fixed or at least most of them. One remained.