Why is v0.21.1 being replaced with my old v0.17 install? This is a Windows 10 installation. Thank you!
Did you download the "exe": https://bitcoincore.org/bin/bitcoin-core-0.21.1/bitcoin-0.21.1-win64-setup-unsigned.exe
or did you download the "zip": https://bitcoincore.org/bin/bitcoin-core-0.21.1/bitcoin-0.21.1-win64.zip
If you downloaded the .exe, then it should have "installed" 0.21.1 into the default location (C:\Program Files\Bitcoin) overwriting any previous version. If you downloaded the .zip and extracted it, it will just run from the directory where you extracted it... any existing shortcuts in the Start menu will likely continue to point at the "old" version in the default installation location.
That is to say, if you "installed" 0.17 using an .exe and it is in "C:\Program Files\Bitcoin"... if you download the 0.21 .zip file, and it is in C:\Users\YOURUSERNAME\Downloads\bitcoin-0.21.1-win64 and you run it from there... then 0.21 will open... but if you use the Start menu shortcut, it will likely start up version 0.17 instead!
As someone else already mentioned, there is also a possibility that something is "broken" with your Windows 10 install, and it is automatically restoring a previous "Restore Point" from before 0.21 was installed.
If this seems to happen overnight, it might be caused by an automatic windows update that is restarting your PC, encountering an issue and then restoring.