I was comparing a couple of files and made some editings. Not many; I think 4 or 5.
Then I changed my mind and decided to revert to the original, so I typed <ctrl>Z repeatedly without counting.
With my surprise, what I got was:
A B C D
A B C
A B
A
-
A B C D
A B C
A B
A
-
A B C D
...
I'm not sure if the cycle passed trough the original configuration (that above I marked with '-') or not but I think it did.
I was in a hurry so, to be sure, I closed EDP (64 bits, 12.0.0.4) without saving and restarted it.
Take this as a warning. At the moment it seems I'm unable to reproduce the problem.