I’ve done:
…about a billion times. Why am I struggling today with this move sometimes working, sometimes failing. What’s the difference and why does this happen?
Sub-divide success:
Sub-divide failure:
Looks like you missed the copy function in the second GIF.
Actually, it looks like you hit option after starting the move but it didn’t take. Is it possible you hit it twice? It appears there’s no guideline on the blue axis after the move.
Yes, note the guide at the axis is different.
It’s repeatable, create a guide, move it without the copy function, type in n/ and you will see the small dotted line at the origin become the larger dotted line of the guide.
Do you need to clean your keyboard or have you worn out the option key? Or maybe you just aren’t holding your tongue right. 
That must be it! 
I must be me slipping up. I always wonder with the Wacom tablet, but then I would repeat it with the track pad too. I am long overdue to move to a new machine, which I already bought.
To verify when you are copying you will see a small “+” icon at the cursor. in the first video it is there in the second it isn’t. It can be easily toggled on and off with the ctrl/opt key (purposefully and accidentally) but keeping an eye on the + should let you know the mode you are in.
