Found out today that there have been changes made to the 19M, and the FBI will be issuing the "19M 2020" in the future. The changes were proposed by Glock, not the FBI, and are considered "running enhancements" to the product line and had to be approved by the FBI.
Going forward, the half-moon cut will be removed and FCS will be added. There are several small changes internally, but the biggest is a design change to the barrel. The locking lug has been made slightly taller, which changes the timing and the barrel tilt. It's supposedly more accurate.
I've been assured that there were no issues with the existing 19M's, and that all the changes were proposed by Glock, not the Bureau.
A new version of the FBI G43 is coming as well.