Hi Triplets,
The standard 100 which was the same in my last computer and never had this problem. Granted, it was an older version of Logic I had been using for quite a while. I don't know if it's the fact that it's literally the latest version of Logic (on a new computer) and maybe it's still glitchy.
In another forum someone posted (in regards to this same problem someone else encountered) "to turn on both "Mixer" and "Plug-in" buttons in the Undo History pop-up. I just did that, so we'll see if it works. Again, it happens at random so it's a matter of trying and seeing if it happens again.