An application which uses the Qt Help Engine is installed to the Mac Applications folder. The .qch and .qhc Qt Compressed Help and Qt Help Collection files are placed in the same directory as the application executable.
When the application is run, on opening the Help Window, the Help Contents and Index remains blank, no discernable error message are displayed or appear to be logged anywhere.
Running the application again, but this time as root using sudo, the Help Contents and Index are populated correctly, after which the qch file modification date will have been changed, indicating that the Qt Help Engine has written to the file.
QTBUG-72174 partially addressed this issue, however, I believe that the maintainer had incorrectly conflated the Qt Help Engine and Qt Assistant requirements.
The Qt Help Engine, by means of the QHelpEngine class, provides a means for Qt-based applications to include cross-platform help.
Having this class modify pre-generated help files included with an application, files which should be absolutely static, should be completely unnecessary and not the expected behaviour for a "viewer".
Qt Assistant, on the other hand, is a tool for viewing similar formatted help files using the same Qt Help Engine. If a requirement of this tool is to modify the content of these files, then so be it. But then, the Qt Assistant developers should be using a subclass of QHelpEngine class suitably modified to suit the application, rather than modify and breaking the QHelpEngine for all other applications.