Hi @seetravers Chris,
I have noticed this before, but what I have found is that if you are not actually inside of the field it will select it all. I have come across this with Rich Text and Plain Text fields.
Id just recommend that your cursor is within the text field before hitting CMD + A.
For plain text the field must be highlighted blue before using CMD + A, for rich text the cursor must be within the field for that feature to work. Refer to my GIF!
Hope this explains. Not really a bug, just how the CMS works!