To reproduce.

 

1- Make some change to content in a cell (A).

2- Leave "edit mode".

3- Press Ctrl+z to undo the change. Change is undone. Fine.

4- Edit the content of some other cell (B); but stay in edit mode!

5- While still in edit mode in cell B, press Ctrl+z to undo the change you made (the change will be undone, fine), and then press Ctrl+y to redo it : It won't undo the last undo in cell B but will silently undo the undo in cell A, without any visual cue, hint of what has changed, etc..

 

So, basically, each time one presses ctrl+y in edit mode, some change is being undone somewhere, silently.