Hello,
There seems to be a problem regarding this missing flag for Push on Android. I'm aware this issue is known and was closed last year( #352 ). But somehow it is still missing in Attach Version 4.0.21.
To test this I have built the Attach modules locally as 4.0.22-SNAPSHOT and the problem disappeared. Maybe there was an issue when building/deploying 4.0.21? I have not tested it with other versions though. Going by the date of the merge into master ( #353 ) branch, theoretically versions 4.0.18, 4.0.19 and 4.0.20 should have this fix.
Please let me know if you need more information.