Date Filter Between Date Odd Behaviour
I had a similar type of issue and managed to fix it by changing back to "Single Filter" and adjusting the filter value type to what I wanted to use in "Grouped filters" type. I then went back to "Grouped Filter" and my date filters suddenly started working.

Alternatively, you can just delete the filter and add it again, and try set it correctly from the start. 

Hope that helps.

p.s. using Views 7.x-3.6