Well, it's definitely there and it's also in the repodata:

https://dl.fedoraproject.org/pub/fedora/linux/development/rawhide/Everything/x86_64/debug/tree/Packages/m/mingw-qt-debuginfo-4.8.7-1.fc25.x86_64.rpm

and dnf debuginfo-install works fine here:
% sudo dnf debuginfo-install mingw64-qt
enabling rawhide-debuginfo repository
enabling rpmfusion-free-rawhide-debuginfo repository
Last metadata expiration check: 0:00:18 ago on Tue Jan 24 16:01:56 2017 MST.
Dependencies resolved.
========================================================================================================================
Package Arch Version Repository Size
========================================================================================================================
Installing:
mingw64-qt-debuginfo noarch 4.8.7-1.fc25 rawhide-debuginfo 142 M

Transaction Summary

Install 1 Package

Total download size: 142 M
Installed size: 973 M
Is this ok [y/N]:

So, perhaps it's something in trying to install it via dnf directly without the plugin?

Does dnf debuginfo-install work for you?

Okay now I don't know whether to feel stupid... First attempt was suspicious because fedora-debug was missing from the list of repos:

$ sudo dnf debuginfo-install mingw64-qt
enabling rpmfusion-free-rawhide-debuginfo repository
enabling rpmfusion-nonfree-rawhide-debuginfo repository
Last metadata expiration check: 0:00:00 ago on Wed Jan 25 00:09:24 2017 CET.
Dependencies resolved.
Nothing to do.
Complete!

Then I did a brute force clean of the dnf and started over, and see there, now it appears. Corrupt metadata I guess? Closing, sorry for wasting your time.

No problem at all. Glad it's working.