In Wayland you can't use KeepassXC's very cool Auto-Type feature (it's somehow Qt's fault?) but if you installed it as a Flatpak you can go into KDE Settings, search for "Flatpak Permission Settings" and in the settings for KeepassXC under "Advanced" you can disable "Wayland Windowing System" to make it work. Nice!
Using the environment variable QT_QPA_PLATFORM=xcb should do the same thing, but it likely won't fix your problem. These two methods allow KeepassXC to run on X11, which lets it access other X11 apps (running on XWayland), meaning native Wayland apps still won't be able to use auto-complete.
There's probably no way around this for now, as this is due to Wayland's design, which has stricter keyboard access safety, as opposed to X11 which just let all apps read/use the keyboard all the time.
It has to be added to the command when you want to launch it, or added to the .desktop file so it does so automatically. On KDE, it should be as easy as right clicking it on the start menu and clicking "edit application" on the second tab there should be a command field, where you can add the variable at the beginning.
In case this doesn't fix it, your alternatives are copying and pasting passwords or, if your main use for it is in a browser, using the official extension.
This is because Wayland doesn't allow it to read window titles. Keepass and KeepassXC uses the window title to identify which entry to use. If you have no title, you can't find the entry. That's why it will not work with Wayland and never will work, until Wayland allows it to read window titles.
XWayland, which is forced with your workaround, is not Wayland.
That's at least for me, the main reason not to switch to Wayland. I have no idea why Wayland doesn't allow reading window titles. There is absolutely no security or performance benefit of this behavior. For me it's either a bug or a design failure. Or simply bad behavior.
Disable wayland means it runs through XWayland. I guess autotype will then also only work in other XWayland apps. Which means that isolation between all these apps is gone.
The old security vs ease-of-use problem. It's kinda a hassle to copy-paste all this logins and passwords (haven't done it in a long time). Switched to Wayland now because gaming seems to be better (had some nasty screen-tearing on Xorg and possible borked my monitor setup while trying to fix it).
Not having autotype is for me the reason not to switch to Wayland, too. I don't only have passwords for websites stored in KeepassXC - otherwise it wouldn't be a problem with the Firefox addon.
Copy & paste of passwords into e.g. the console can't be better security. I wouldn't mind if autotype would work in general and one has to choose the corresponding entry manually without automatic matching to the current window. But even autotype into the last active window doesn't work.