[KDE neon 6.3] I have to wait for the login screen to black out and reappear before logging in

1. I start my computer ;
2. The loading screen appears ;
3. The login screen appears (but logging in will not work) ;
4. The screen turns black with underscore ;
5. The login screen reappears (logging in will finally work) ;
6. I'm logged in.

It has been like that for a while, I expected this bug to be found & fixed eventually, but after running an update today, it still hasn't, which is why I'm now posting here.

So, when the login screen first appears :

• either I have time to type my password & press enter, but afterwards the login screen still disappears then reappears as if I never tried to log in ;
• or I'm still typing the password while it disappears then it reappears as if I never started typing ;
• or I wait for it to disappear & reappear, and only then I can finally log in.

What to do ?

Thank you