SUMMARY
I find this rather difficult to explain in plain text and can't record the screen because wayland, so I'd do my best.

Setting globalmenu in button factor (where it shows itself as a three-bar icon, you click it and the app menu appears) don't work as expected with Wayland. You can move your cursor through the elements of the first level menu (i.e. File, Edit, Preferences...). The second you move the cursor from one menu item to other, the menu autocloses itself, so you have to click the globalmenu button again and go to the desired menu item, being careful enough to not hover over any other menu item.

Also, the second-level popup menu (say, for example, the menu items contained by the 'File' menu) shows at the absolute center of the screen. There's a obscure pattern there too, because if you move the cursor in some direction the whole menu will autoclose, but in other way it does not - can't explain how, because I'm not quite sure of how that pattern works.

Weird enough, globalmenu works as expected when clicking the globalmenu item and activating the menu items with the keyboard (with cursor keys) or *almost* as expected when setting a global keyboard shortcut for it and activating it via keyboard. Second-onward level menus appear where they're supposed to show up, right next to the first level menu, but when setting a global keyboard shortcut and activating globalmenu with it, sometimes the first level menu will show up with a weird layout where the items show up with center-aligned texts and are not activatable/hoverable.

STEPS TO REPRODUCE
1. Add globalmenu to a panel, whether horizontal or vertical, and set it to button factor.
2. Open an app that has appmenu support (say, Dolphin).
3. Click the globalmenu button created in the panel.
4. Hover over a menu item and, if said menu item has a secondary menu, move the cursor to it.
5. Hover over several menu items on the first level.

OBSERVED RESULT
If (4) has a secondary menu, it will show at the absolute center of the screen. Trying to reach it with the cursor is difficult - if you move the cursor a certain way, the whole menu will close automatically.

On (5) the whole menu closes automatically, not letting the user to walk through the secondary items of a menu - you'd need to click globalmenu again, and carefully place the cursor over the desired menu item without hovering over any other item, or the whole menu will close automatically again.

EXPECTED RESULT
The user can walk through the items of any given menu without needing to activate globalmenu for each menu item, and secondary-onwards levels should show up right next to their parent. As traditional menus do.

SOFTWARE/OS VERSIONS
Linux: Gentoo x86-64, Kernel 5.12
KDE Plasma Version: 5.22
KDE Frameworks Version: 5.82
Qt Version: 5.15.2