How is this the only thread I can find on this matter?
This has been happening with me for quite some time over two different devices, and now my Mum has just mentioned her phone is doing it too.
Only with Messenger and only when sending via SMS.
Welcome to Android Central! There are actually many threads about this. It appears to be a problem with sending MMS via FB Messenger on Samsung phones. Change your default SMS/MMS app back to the stock Messaging app, and send feedback to the FB Messenger developers about this problem.