It seems that check numbers are not retained during sync.
When I enter a check transaction in Quicken Mobile 5.16.2 for iOS, the check number is lost after the entry is synced. When that transaction is transferred to Quicken Mac 5.12.5, the check number does not appear in the transaction. On Quicken Mac, the check number is retained after sync. The check number does not get transferred to Quicken Mobile, however. This is a recent problem that I first saw today (9/28/19), but not two weeks ago. I think that there have been new releases of both Quicken Mac and Quicken Mobile during that time. It seems that the problem is more related to Quicken Mobile than to Quicken Mac. Finally, this problem happened with at least 5 checks, so it appears to be a consistent problem.