Not a regression. Found on 1.22 (build 217). Reproduces on 1.21.
1) Open iOS app
2) Open any Channel or DM
3) Tap on + by the text box and use Take Video to record or Photo Library options to attach/post an existing video to channel (using your iPhone)
4) Tap on newly posted video to see preview
5) Tap on Download link in upper right
Expected: Download prompt
Actual: Download prompt is missing, user is presented with the Save Video Error. Error can be dismissed, but user is left without any way to download.
Using Delete Documents and Data option in Advance Settings appears to clear this up and video can now be downloaded before saving.