Just download Android Messages by Google and stop using Samsung Messages
To the Samsung, just compare your app to how Android Messages sends gifs and you'll see what everyone is talking about
This happened to me on my S8+ and my Note 9 (both on tmobile), it's true it's only some gifs, and clearing the cache does nothing, also this 'advanced messaging' suggestion does nothing.
It's much better, but I have a Galaxy watch and Samsung messages is required.
All of Samsung's apps are so close to being really good, and this long term issue is a reason why people don't like them