MySQL workbench alter table is incomplete
There seems to be an anomaly or bug when a timestamp column has a default value of CURRENT_TIMESTAMP() - with the parenthesis. 5.7 documentation says it is synonymous with CURRENT_TIMESTAMP but workbench must disagree - sort of. 

Changing the definition in workbench by removing the parenthesis provides a message that nothing changed. However changing to "CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP" fixes the problems mentioned in the question. And subsequently changing it "CURRENT_TIMESTAMP" only also works.

I'll report this as a workbench 6.3.6 bug.