Why isn't the new Facebook Messenger SMS/MMS feature sending MMS?

I really like the new Facebook Messenger SMS/MMS feature, although I may be in the minority. I like having everything in one place, and I like its simplicity. The SMS sending and receiving works properly, and I am receiving MMS messages (group texts). But when I send MMS messages, they aren't received by anyone in the group text. Any ideas?

I have an LG G3. Android Marshmallow (6.0)

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