When updating an existing project, platform sources are not exported again by default. Use this flag to re-export platform sources. This will overwrite all the platform parts of the existing project, including the exported platform sources.
qmlprojectexporter my_project.qmlproject --update-project main_project.gpj --export-platform
This option was introduced in Qt for MCUs 2.6.
Available under certain Qt licenses.
Find out more.