Solved!
The Playstore review report in Open Testing mode revealed the content of the blank messages.
The flash screen message was to do with Write permission for Android versions below 11.
I solved this with the extension - How to request and grant MANAGE_EXTERNAL_STORAGE permission
Also, used the Utility block to see if version is <11 and presented the Write permission request for these devices.
The second set of blank alerts came on the homepage - these were to do with my using -2 for the Width percentage attribute, instead of the plain Width attribute.