Presumably this:

- but we have no details as to how or why the download fails.

If this is because of an erratic connection, using a tool such as Free Download Manager can help.

Google -> e.g.
Download Microsoft .NET Framework Repair Tool 4.8.4072.0
- which ends up downloading from MS...

... note the suggestion here:
How to repair windows10 system disk when MS tools fail-1.png