I recently upgraded from an iPhone 6s+ to iPhone XR purchased directly from Apple. (my service is through Telus, obvs )
For some strange reason, I can't seem to send a GIF as a MMS message. Had no issue with this on the 6s+.
The following all work:
-sending GIF via iMessage to another iPhone user.
-sending a photo via MMS to an Android user.
-sending a video via MMS to an Android user
However, if I pick a GIF, either one that I choose in the iPhone's #images search, or via Giphy that's integrated with the keyboard I use, and try to send it to an Android user (I've tried both with my wife, also a Telus subscriber, as well as to my sis-in-law, who uses Rogers), the message always comes up with "Not Delivered".
Chances are once had changed the SIM it was being sent as an RCS message vs. a standard MMS. RCS is designed to be more like an iMessage or WhatsApp message and will allow file transfers where a standard MMS would be limited to either a standard image or having to send a video clip.
This issue seems to be fixed in the iOS 12.2 beta 2 that was released this morning, so it seems like they CAN address this issue. I'm not sure when 12.2 will officially be out, but the beta can be installed by visiting https://beta.apple.com/sp/betaprogram/