microsoft word - How do I find a rogue keyboard shortcut?

Sometimes, I highlight text in an MS Word document and wish to add a comment about it to send to a co-author. Following this official documentation about keyboard shortcuts (in the “Review documents” section), I then press Alt+Ctrl+M as the keyboard shortcut to insert a comment. I usually use the modifier keys on the right side of the keyboard because they're close to the M and I can easily cover all three with the hand that didn't just use the mouse to select that text. To within human perception time, I often press both modifier keys simultaneously, before the M (so I don't type an m).

However, about half the time, instead of inserting a comment, the highlighted text is deleted and replaced with a lowercase mu character (µ). After years of being unable to figure out the pattern, I have finally found that on the right side of the keyboard, if I press Alt+Ctrl+M (with the Alt key going down first) I get the µ, and if I press Ctrl+Alt+M (with the Ctrl key going down first), I get the comment. If I use the modifier keys on the left side of the keyboard, the order doesn't matter and I get the comment either way.

When I right click on the Ribbon and choose "Customize the Ribbon"→"Keyboard Shortcuts," I can see that the shortcut to insert a comment is listed as Alt+Ctrl+M:

Although the modifiers are listed in the wrong order, this matches the Office documentation.

When I go to the Insert→Symbol→More Symbols dialog, it says the shortcut key assigned to the mu character is Alt+X, nothing like Alt+Ctrl+M:

EDIT (thanks to Jonno for these questions): My Windows keyboard layout is US English. The Ctrl key can be omitted in the right-side-Alt+M combination and I still get the µ character. However, right-side-Alt+M (with or without Ctrl) does NOT produce the µ character in other Windows applications such as Notepad, WordPad, Firefox, or Chrome. Therefore, I am doubting the "system keyboard mapping" theory (thanks Jonno!) that previously seemed promising.

There may have been some time in the past where I needed to type a lot of µ characters, but that time is not the present, and it's annoying to accidentally delete some portion of a document and replace it with an unexpected character, especially after most proofreading checks have taken place and shortly before that document is formally submitted.

Why am I getting µ characters for Alt+Ctrl+M or Alt+M (but only with right side modifiers, and not for Ctrl+Alt+M), and how do I get rid of this behavior?