This might be connected to the addon or not, but I have the
Moar Units Mod installed, and when I researched the tech for Tanks, my production queue for Cuirassier (German replacement for Cavalry) somehow bugged out in some cities. See the attached screenshot, the queue for Heidelberg says there are Panzers now (German replacement for Tanks), however the icon in the city bar says that nothing is currently produced. It also doesn't immediately open the production overlay when clicking the icon, I needed several clicks for whatever reason.
To make this work again, I need to remove every Panzer from the list (can't remove the last entry, so first have to insert something else in the queue, and then delete the Panzer entry) and add them again.
Note that this also happened for other cities (Mainz, which I've already corrected in the screenshot, and Köln, which is not yet corrected). However for other cities, the switch to the upgraded unit worked fine (e.g. Trier and Aachen).
// Edit
This might actually be connected to the fact that the cities were it bugged out just had constructed a Cuirassier/Panzer on this very turn where the upgrade tech was reasearched. The other two cities were still in the middle of production.