I did a few tests and apparently the source filter for wmv that comes with windows is broken(in windows 10 at least) for the original file but not the remuxed ones so if you don't have the internal filters for mpchc, that's probably your issue.
So if you see "WM ASF Reader" under filters if you right-click on the video window, that's why it has broken seeking, not sure why exactly though, wmv files are often complicated.
You can either fix it by installing LAV filters or installing a version of MPCHC that comes with the internal LAV filters(the exe installer comes with it but the archive version may not unless you also get the standalone filter pack).
And if you have a mix of windows and LAV decoders and are experiencing slow seeking, you can fix it by going into the options, in the external filters and adding the windows one and setting it to block. After closing the video and re-opening it, it should not use the filter you blocked and will seek fine.
You can also get an up-to-date version of MPCHC here:
https://github.com/clsid2/mpc-hc/releases
The original team discontinued the project because of a lack of developers interest but some guy is keeping it alive with small bug fixes and stuff like that.
I'm still downloading one of the files uploaded by mickey_kenny just to make sure it's the same file as the original post since it was renamed and I'll test it to make sure but it seems to be the case from the file size.
Edit: Finished downloading it and they're almost identical so that's not an issue with his files. Somehow it has an extra 0 at the end but that's not something that should affect things other than waste a tiny bit of space.