Initially, I cannot understand your assertion, in particular that “nobody has been experiencing this”. Why not? No KDE user uses the Calendar app of Kontact (one of the first KDE software)? It would be very strange. Or maybe no KDE user sets reminders on their appointments / events? I think, that is unusual, even if possible.
So I thought, I test it with the newest Kontact release.
Thus, I installed the current KDE neon OS on an old Notebook, but I cannot test it: every time I tied to put in an event the app crashed. The OS was also very unresponsive. This happened not only on my old Notebook, but also on the newer one and on the Desktop PC (64 GB RAM) with the live OS version.
I am not able to determine, if this is due to the application itself or to a system lack, but in any case I was then able to understand the second part of your assertion: “nobody has yet had time to try and reproduce it.”
KDE OS stability has obviously first priority!!
Now I ask me if I have to:
repeat this thread about every month until I get an answer,
report a bug,
simply forget this, because there are few possibilities to solve it in the next future.