you could try the new testing version: 🧪 Help testing the upcoming 1.33.X release
Also make sure you haven’t disabled notifications for DC in your KDE settings, I guess?
Do other apps / electron apps send notifications successfully?
when you start DC via the terminal, are there any messages about libraries missing?
Also you know that you only get notifications if the DC window is not focused / in background, the chat is not muted and the account is selected.
It should generally work on KDE, speaking both from own experience and from what I heard.