I've seen some info online regarding this, but none of the solutions have helped. Using Office Professional 2021 (installed from disk). Starting today, while Ctrl-i and Ctrl-u work fine toggling the state of a single word, Ctrl-b toggles the entire document (and back on second press). Style is Normal. Happens regardless of font.
Done the following:
Local repair of Office
Recovering archived copy of Normal.dotm from before
Ran Word in Safe mode -- problem did NOT manifest!!
Since Style=Normal, there's no issue regarding whether "Automatically update" is checked or not
Looked in Com Add-ins -- nothing identified as "active or not"
HELP?

Also tried putting "ActiveDocument.Styles(wdStyleNormal).Automaticall yUpdate = False" in Immediate Window in Visual Basic Editor, pressed Enter, and saved Normal (from another online answer). Problem still there on next opening of document.

A couple more things which may aid in diagnosing the issue:
1. When, for example, a single word has been selected, and Ctrl-b (or "B" on the ribbon) is pressed, the selection expands to the entire document. This selection persists even after either pressing Ctrl-b a second time or Ctrl-z.
2. If I select a word, go into the Font dialog box, and select Bold, just that word changes state.
Hoping to hear from someone with more things to try.

It sounds like you have a macro associated with the Ctrl-B keyboard shortcut.

Does it happen if you start Word in Safe Mode?
Does it happen in all documents or just one?

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia

No macro -- Ctrl-B is assigned to "bold" in the "Customize Keyboard" - "Home Tab". Using Normal style, occurs in all fonts. Italics and underline work fine. This just started two days ago right in the middle of editing a document! And, again, upon doing Ctrl-B to a specific selected word, the entire document becomes selected.

Because you are not seeing the problem in Safe Mode this tells us that you do have a macro associated with the keyboard shortcut.

The setting could be in the document itself or any of the active templates (Normal, Attached or Addins). You would get a summary of active keyboard customisations if you go to the Print Dialog and change the settings dropdown which says "Print All Pages" to "Key Assignments"

Once you find out which file contains that setting, you can either change the template or go into the Customise Keyboard settings and change the setting in that template

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia

Created totally new document in Normal style. Tested -- found Ctrl+B misbehaved just as always. Did Print/Key Assignments. Ctrl+B reports: "Bolders [sic] the paragraph between the indents"
Opened the document I was working on when this problem first manifested itself the other day. Key Assignment report shows identical thing for Ctrl+B.
HOWEVER -- when I went to File/Options/Customize Ribbon/Keyboard shortcuts, I found Ctrl+B assigned to a "bold" macro, not the Home "bold"
Removed assignment. Went into macros and changed name of "bold" macro. Went back to document -- Ctrl+B now behaving. Created another new document. Ctrl+B behaved!
SO -- even though the key assignment printout didn't show there being a macro involved, your comments led me to check that other document, and find it to have been the culprit. So -- MANY THANKS!