The download page says the file should be 1.25GB, yet the actual download is only about 911MiB – too big a difference to be a GB vs GiB difference (though I believe your GB to really be GiB in any case, that is, binary GB, not decimal).

Further, it seems that a large chunk (about 20 minutes) of the
movie, relatively near the start, is simply absent, as if perhaps,
300 MB or so had somehow been dropped from the file.

So, could you possibly check it, and perhaps upload it again if
that seems like a solution?

Thanks again.