You might have to wait for an updated Windows 10 forum-user who also uses MS Word to come along.
It would help if you give the version of Ms Word. Is it the cloud based version? The fact that you can paste into WordPad is suspicious.
I normally use linux and only have a very basic, not updated Win 10 virtual machine (VM) and no MS Office installed.
However, I was looking at various things this morning concerning Gimp and copy/paste to other applications and really the Win clipboard is very poor.
Have you tried a third-party clipboard such as CopyQ, it might not work with the latest-greatest Win10 but worth a try.
There is a portable version.
https://www.portablefreeware.com/index.php?id=2900 easy to dump if no good.
This is what it looks like here. screenshot
https://i.imgur.com/3fL9uNT.jpg
When it comes to MS Word, I appreciate that with certain organisations, especially academic, MS Office is virtually mandatory, however LibreOffice (LO) is normally more than a good substitute.
This pasted into LO
https://i.imgur.com/bVf7BEK.jpg
You could try asking the question on
http://gimpchat.com/ where there are Win10 users who have certainly had their share of update problems recently