Thanks Martin, that is indeed the problem as I actually wanted the bookmarks mode only not location profiles.

I have now ensured it defaults to bookmarks mode and it now works perfectly.

Thanks again.

Well, a location profile is a pair of local and remote directory. If your location profile has local directory empty, then selecting the profile will indeed clear the local directory box.
Are you really using the location profiles (the pairs)? Maybe a simple bookmark mode would suit you better:
https://winscp.net/eng/docs/task_navigate#bookmarks

WinSCP 5.21.5.

When I select a different remote directory from the Site Location Profiles it clears the local directory field which if accepting results in the local directory switching to the C drive.

The attached demonstrates what happens.

bookmark1.png - open bookmarks.
bookmark2.png - current directories are displayed.
bookmark3.png - A remote directory is selected - in this example /usr/shared - from the Site Location Profiles which results in the Local directory field being cleared.