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I have noticed this issue in Dolphin, Kate, and transmission-qt during the past few weeks. Other applications work fine. This makes me think that it is an issute with Qt. When I right click something or press the menu key on the keyboard, no context menu appears. Similarly, if I try to open a menu from the menu bar at the top of the application (the File Edit View thing), if present, the button gets highlighted to indicate that the menu should be open, but the menu does not appear. However, in both cases, although the menu does not appear, I can still interact with it as usual if I remember the exact position of the item I want and click there, press the key to trigger the function that I want (not sure if there is a term for those), or navigate with the arrow keys and hit enter. Essentially, the menus function 100% as normal but are completely invisible. This makes Qt based software extremely difficult to use. A workaround is to run my Qt applications on xwayland using QT_QPA_PLATFORM=xcb. On x, the issue does not occur.
I could not find much information on this issue for some reason. This bug report and this bug report only seem loosely related, in the sense that they involve invisible menus. The first applies only to Windows systems, occurs randomly, and is fixed by restarting the application. The second occurs only after minimizing the application. Both are old and look to be resolved. In my issue, the menus never appear (even immediately after opening the application). This looks closer (though the initial comment is kinda confusing so I don't know for sure), but that bug is almost a year old and was fixed in 5.15.1, and I'm on 5.15.2-5. Due to either the lack of information or my inability to find it, I don't really know where to start with this. Is it likely that I have a configuration error, or should I try downgrading packages?
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I have similar problems with some programs.
Master PDF Editor (https://aur.archlinux.org/packages/masterpdfeditor/) for example is one that I remember at the moment. Build 5.7.40 uses Qt 5.9.5, which was released in April 2018.
I guess unless there is some way to add compatibility for older Qt versions to wayland, you have to wait for the app maintainers to use more recent Qt versions...
As you already said, the workaround to use xwayland also works: QT_QPA_PLATFORM=xcb
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