I put the main panel at the top instead of the bottom.
It used to work fine but I had to reinstall on a new nvme drive because my other one suddenly died (smart thinks it’s ok but I can’t write to it).
So new install of 23.04/updated.
The problem is when the main panel is at the bottom it is all ok.
I’d check you don’t have two panels… with the empty panel previously covered by the panel you’ve now moved to the top.
I’d likely treat it as such; ie. add a widget, menu or something to confirm the system is treating it as a panel, then just remove it. Your alternative is exploring ~/.config/lxqt/panel.conf too, as that would confirm etc too, but you can do it using the LXQt gui.
I’ll add that should you not detect it as a panel, ensure you’ve logged out & logged in again, as I’ve found LXQt can get itself lost on rare occasion (1.2 & above) & logout/login fixes this.