Reddit is the only major platform in this catalogue that hosts animation two different ways at once, and nothing on the page tells you which you are looking at. An item uploaded directly as an image file can remain a genuine GIF on i.redd.it. Anything routed through Reddit's video pipeline ends up on v.redd.it as an MP4, looping silently so it reads as a GIF to anyone scrolling past. Both look identical in the feed.
The reason for the conversion is size. GIF dates from 1987, compresses poorly, and stores only 256 colours per frame. A few seconds of smooth animation can reach twenty megabytes as a GIF while the same clip as H.264 might be under two, and look noticeably better. At Reddit's scale that difference is enormous, so large uploads get converted automatically.
This explains the confusion people run into. Save one Reddit GIF and you get an animated image; save another and you get a video file, with no obvious reason for the difference. Nothing has gone wrong in either case — you are simply seeing what Reddit actually stored for that particular post.
If you need a specific format, converting is possible in both directions and the trade-offs are worth knowing. Going from MP4 to GIF costs colour depth and multiplies file size several times over, because those are properties of the GIF format itself. Going the other way, from a source GIF to MP4, shrinks the file dramatically and improves gradients. For most uses the MP4 is simply the better file: Slack, Discord and most modern chat applications loop short silent MP4s exactly like a GIF.