SUMMARY
If you click on any of the options in the menu bar of this Java 11 app called IRPF 2024 (found on Flathub), while its window is maximized, KWin immediately closes the menu, instead of keeping it open.

This behavior started happening with Plasma 6.2.0, but it was fine in 6.1.5, so I ran a git bisect between these two versions, and I found the culprit commit:

https://invent.kde.org/plasma/kwin/-/commit/7b486e215d44bbd9fdeb8f1180c76fb16ec6a311

STEPS TO REPRODUCE
1. Install the Java app from Flathub: flatpak install flathub br.gov.fazenda.receita.irpf2024
2. Run it: flatpak br.gov.fazenda.receita.irpf2024
3. Maximize the app's window (important)
4. Click on any of the options from the top menu (e.g. "Declaração")

OBSERVED RESULT
The menu briefly opens up, but KWin closes it immediately.

EXPECTED RESULT
The menu should be kept open.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.2.0
KDE Frameworks Version: 6.7.0
Qt Version: 6.8.0
Kernel Version: 6.11.3-arch1-1 (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i5-3570K CPU @ 3.40GHz
Memory: 7,7 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 660/PCIe/SSE2