Cell contents shifted in part of one column on their own?

I have an old and large (60k+ rows across several sheets) Excel 97 sheet in which I just noticed something odd. In one column, about 23k rows down out of 48k, all the data in the cells shifted up two rows. The majority of the cells in this column are blank, so it was easy to miss and I don't know when it happened.

I'm not asking for how to fix it -- I don't know how much data was added to this column after the shift, so I can't just move everything back down in a clump. I'm going to have to move it all back down manually (luckily it's not critical data).

I'm just wondering how this might have happened. Does it sound like any known bugs? This is Excel 2016 btw yes I still use it. thanks...