qmlimportscanner doesn't find custom qml plugins on iOS platform and therefore they are not deployed to device.
Steps to reproduce:
1. Build the attached plugin project
2. Change the paths in the application project to point to correct location where the plugin is.
3. Build & deploy the application project to iOS hardware
Workaround which can be applied in .pro file
QML_IMPORT_PATH += "PATH/TO/QMLPLUGIN"
QMLPATHS += $$QML_IMPORT_PATH
This works without the workaround above on Android, Windows, Linux and macOS