Voice dictation missing in Messenger on Lumia 950, but Cortana dictates

[Windows Central Question]

I've noticed that any time lately I've tried to use voice dictation in my Messenger app, there's simply no microphone icon there for me to use when I bring up the keyboard. Cortana is working fine, and will dictate just fine. In my Speech settings, I have English (United States) set.

What could possibly be the reason I'm not getting a microphone icon for dictation in Messenger?

Lumia 950 on Version 1607, OS build 10.0.14393.67