Again thank you for sharing your feedback and ideas. It is highly appreciated!
There are more factors to production times than just the level of the production building. Morale in the provinces are likely to be different as well, which will alter production times between those provinces as well.
In the end it kind of comes down to a matter of consistency I suppose. Right now, when you select multiple provinces the production time will not be displayed. If that was different and production times would be displayed in those rare cases where all factors are exactly the same and production times are indeed equal, players would likely be confused why it is displayed sometimes and why it is not at other ocasions. Or they start wondering why is it not displayed? What is different? But don't get an indication displayed either.
I fully agree that it would be great to have the production time displayed when selecting multiple provinces, but that needs a proper and consistent solution in terms of display space, usability. Until a feasible solution is found there, I think the current setup works best, although not perfect.