The behavior you describe is not standard for Windows AFAIK. Do you, by chance, have the adaptive sticky keys turned on?

I have never seen this. However it could certainly be a matter of timing.

What happened is that Word since 2007 has been using the Alt key as a trigger for screentips for commands. I will often use one thumb for the Alt key and the fingers on the other hand to type the letter.

It could also be a matter of sensitivity of your Alt key. Try using the other Alt key and see if it makes a difference.

I feel your pain. I recovered from CTS after much therapy. I also used a wrist support and brace for years.

If you do much typing look into the Dvorak keyboard layout. I've been using it for about 30 years now.

Yet one more possibility would be that one or more Add-Ins is intercepting the keyboard.

Try starting both Windows and Word in safe mode and see if that makes a difference. (Note, this is a diagnostic, not a solution. If it makes a difference, it gives us places to look.)