Not a bug. Not the wrong  mode. That's just how it works.

It doesn't try to keep the previous contents and do a comparison. If
you take a cell  into edit mode  and then leave the cell without Escape,
the cell is considered changed.

There are indeed times when doing that will change the value of the
cell. E.g. if you have somehow got a text version of a date into a
cell, then doing this will get Excel to re-evaluate the cell and treat
it as a date.