Not sure whether this must be broken into 2 issues..

• create simple Qt Widgets Application from template
• right click on the Forms node and trigger "Add New..."
• choose "Qt" -> "Qt Designer Form Class"
• follow wizard instructions and ensure the new files are added to the created project

• files do not appear on the project tree
• check pro file - files had not been added here [issue 1]
  • add them manually

• files still do not appear on the project tree [issue 2] (macOS - but they appear on Linux)