But as it turns out, there was a problem with the update. Those who loaded it up on their phone either via an OTA update or via Kies, found that it slowed download speeds while decreasing LTE coverage. And while the problems with the update weren't affecting every Galaxy S5 user on T-Mobile, enough of them were having issues that the carrier put the breaks on the update.
So T-Mobile now has to send out an update that not only corrects the original messaging problems, but also deals with the problems created by the update itself. As soon as we hear anything about the new update, we will let you know.
source: T-Mobile via AndroidCentral