Ooops; I spoke too soon. Now the Ctrl+- shortcut inserts a minus sign with spaces around it. This may have been because the minus sign confuses the shortcut somehow. I changed the shortcut to Ctrl+d, and that seemed to work immediately after I assigned that shortcut, but after a couple of unrelated edits, Ctrl+d resulted in the space-minus sign-space again.

I'm using the latest version of Windows 10, and Microsoft Office Home and Student 2021. The Word version is Microsoft Word 2021 MSO (Version 2202 Build 16.0.14931.20118) 32-bit.

An update: the document is in Times New Roman, but the space-minus sign-space was in SimSun (the document was created by a Chinese scientist). I was able to correct this in the Symbol dialog by specifying Times New Roman in the Font pull-down, then General Punctuation in the Subset pull-down, then selecting the en dash symbol, then using the Shortcut Key button to specify the shortcut key combination.

I'm crossing my fingers, but the new symbol assignment seems to be robust.