Windows 10 Pro 1803 build 17134.345
Quicken Deluxe US 2019 R15.16
This problem has been introduced between R14.27 and R15.16 and affects bank and cash accounts, the issue is present in both 1 line and 2 line modes of the register.
With the register sorted in ascending order the New Transaction line is at the bottom of the screen. In R14.27 the icons for adding a flag or attachment to the new transaction are displayed:

In R15.16, those icons are missing from the new transaction line, it looks like this:

As you can see, if I hover the mouse over the place where the icon should be, the pop up message appears, and you can click on the invisible icon and it all works as expected, but the icon is never displayed for a new transaction.
If the sort order is changed to descending date order, the New Transaction line moves to the top of the screen and the icons for flags and attachments are displayed correctly. Switch back to ascending date sort and they disappear again.