I have verified there is a bug with Maskgroup. To generate a MWE (minimum working example) download the example from here https://sdkdocs.roku.com/display/sdkdoc/MaskGroup
1) Edit MainScene.xml and remove the animation block entirely.
2) zip and view in FHD (1920 x 1080)
3) Change Roku display type and view in HD (1280 x 720)
notice the mask coverage is different. This is a bug and renders Maskgroup unusable if the intent is to support 1080 and 720 dynamically.