If I view a calendar and I select "month" then "day" and "week" are greyed out and I can't switch back. I eventually worked out I could get these views back using view:agenda but this makes no sense to me and I guess it isn't very intuitive for anyone else. So... bug or request up to you but the day and week buttons should still work in month view and so should clicking on a day or week in the LHS calendar pane.
Same problem / workaround with KOrganizer 5.10.2 (Manjaro)
*** This bug has been marked as a duplicate of bug 311500 ***