Voice to text messaging has become a pain. When responding from the notification. You get this screen.
[Here](https://imgur.com/gallery/CBzVax8) If you scroll all the way down past all the suggested messages. You have the option to open app. Then your greeted with the normal setup of 3 dots (emoji, mic, keyboard) I just downloaded Google keyboard to try to remedy this. Now when you tap the message field, the keyboard opens and then you can press the mic.
Edit: Just to add upon further inspection. If you tap in the gray area with Samsung keyboard being the default. You'll notice a small horizontal line appears under the t9 keyboard that you can drag up. Once you do that you'll find more options such as microphone to be set as default.