I've found that the ZISROUND function doesn't work correctly on the mobile (Android) app preview/emulator. I have 3 watch faces that use this function where the shape of the white frame changes to match the shape of the watch display. They all work fine in the web creator tool and on watches so must be a bug with the mobile app code.
Phone: Google Pixel 3
O/S: Android 9 (Patch: 5 May 2019)
Facer app version: 5.11_101043-(101043)
Here's an example: