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.