The changes in !5803 (merged) implicitly added the first use of the AUTOUIC_SOURCE_GROUP property by calling cmQtAutoGenInitializer::AddGeneratedSource() with this->Uic. I believe these changes tidy up a couple of aspects relating to the new property.
I initially looked into the property as it didn't seem to be working in the MWE for another issue - the generated headers were ending up in the Header Files source group. This turned out to be because the *.ui file was in the same directory as the CMakeLists.txt creating the target and so the calculated relative path was empty and a path containing // was being added to the source group - this then didn't match to the file.
As a result of looking into the AUTOUIC_SOURCE_GROUP property I noticed it wasn't documented at all so I've also done so here.