This video is HEVC video in the MP4 container. Support is spotty across the web as it is a proprietary format HEVC/H.265 video format | Can I use... Support tables for HTML5, CSS3, etc
The OP is just H.264 in MP4, which is widely supported MPEG-4/H.264 video format | Can I use... Support tables for HTML5, CSS3, etc
The first reply has a video that is AV1 in MP4, which is not supported in Safari on devices without hardware decoders, per AV1 video format | Can I use... Support tables for HTML5, CSS3, etc.
This is not a Discourse bug, as Discourse doesn’t do any video transcoding, only serving video the same way it was uploaded.
One of our teams is working on adding transcoding capabilities to Discourse, but that will need a somewhat expensive AWS integration