When using Field Calculator with Enable Undo is turned on, even after the edits have been saved and consequentially the editing session ended, upon applying another Field Calculation the Enable Undo button is toggled back on, even though it is turned off before clicking Apply.
The Enable Undo toggle button stays disabled/grayed-out until edits are saved or discarded. You cannot change the state of Enable Undo while there are pending edits. Once edits are saved or discarded you can choose if the next run of the tool can be undone using the toggle. Issues with the state of the toggle button after saving edits are not reproducible in ArcGIS Pro 2.7.