@anarcat I’m not sure what was changed in the most recent Debian package, but I see the status message “Use python3-sip module, not the bundled copy of PyQt5.”
PyQt5 >= 5.11 uses its own private copy of sip because it must, according to the PyQt 5 developer. See PyQt5-sip · PyPI
The code in Rapid Photo Downloader 0.9.12 works on the latest versions of Arch and openSUSE without problems. As far as I know they do not need to patch anything in PyQt 5 or Rapid Photo Downloader.
If Debian is forcing PyQt5 >= 5.11 to use the the copy of sip that is not specific to PyQt, then I anticipate that will break PyQt5 in unexpected ways.