I found out this morning that I had this problem on my S8+ - AT&T. The only person I was missing texts from was my girl, not good.
She is also on AT&T using an S5 Active.
I went into the stock app, turned off the ADVANCED MESSAGING features as indicated above -- all is right with the world, and Textra. She can't see that I am typing a response to her now (that seems so iPhone-ish anyway) but everything else works fine.
The stock app is awful, so I am VERY happy to find such great help here, and continue using Textra Pro!
Now if I could only get to the bottom of that outline font problem... hmmm... anyone else see that? All my messages show up on the main panel of Textra in OUTLINE font, hollow characters. It's not a Textra problem though (specifically) since I also see that same font in other places (Lyft, Android Auto). So its something with the GS8+ and AT&T.
Onward and upward!