What messaging app are you using?
Have you checked to see if your messaging app is being put to sleep or deep sleep by the OS? Settings>Battery and device care>Battery>Background usage limits>Sleeping apps / Deep sleeping apps If you find your app here remove it and see if your issue improves.
Are you using a data saver, memory saver or any kind of booster apps?
Does this happen for all messages or just MMS?
Does it happen the second message if you send two in a row?
