Slicers - Dates Between

Hi ecarroll , 
The direction of the relationships would determine the values in your slicer field. 
If the 'other' tables can filter the calendar table then obviously the values in your calendar table will be affected. 

1. Look into DAX functions like ALL() or REMOVERFILTERS that in a measure context to remove any existing filters outside the measure. 
2. Fix those relationships. Determine the direction of filtering between calendar and 'Other' tables. Have your calendar date table central hub for filtering other tables. 

3. Look into 'Edit Interactions' between visuals to see if any existing selection from another visual...could be another slicer synced to the page... hidden filters... hidden slicers...or simple another visual filtering the values in your date slicer. 

Without looking at your data model it's difficult to figure out why the between slicer could be behaving that way. Feel free to tag me (@) in your reply that includes more information and screenshot possibly to better understand this problem. 

Hope this helps, tag me (@) if you need more help on it. Feel free to mark the answer as a solution if it helped you.
Thanks!