As near as I can tell, dnf won't ever download
data from repos merely because of an update command
(no matter how far out of date the metadata is).
This seems like an "improvement" they must have
made recently.
Looking at the makecache timer service, it seems
like you now need to use two commands to download
updates manually at a time of your own choosing:
dnf makecache
dnf update
That worked for me when the dnf update by itself
didn't.