Hi @dspringer ,
To answer your question, I think it may depend on a few components. One component could be due to how the visual types are interacting with your data. As you stated your table visual works and that could be due to how table visuals show all rows of data exactly as they exist. For pivot tables, they do include some visual logic in the sense that they do apply both aggregation and grouping to the data. Another component could be how the parameter interacts with the pivot table logic that results in hiding the December data.
There can be a couple of reasonings to why you are getting this output. Since you know the visual works in a table visual type, then you can go that route and not do it as a pivot table. However, if you really want to do this visual as a pivot table then you can either upload your dashboard to the Amazon Arena Sandbox and I can take a further look into it (refer to this link for more information on the arena: Amazon Arena Sandbox: An embedded instance of the Quick Sight capability, within the Amazon Quick Community - What’s New - Amazon Quick Community). Or if you do not feel comfortable with that, then can you provide more details into the fields and parameter you are trying to use.