Not so much a bug as unclear controls.
The "Needed Placed Dates" tool searches for the dates needed to fill the calendar (on the first loop by default but paying members can look for other loops). The filters are filters on the displayed caches to use to fill, not on the calendar state being checked.
For example, let's say I'd found Traditional caches hidden on each day in January, mysteries hidden on each day in February and multis hidden on each day in March, with no finds in any other month. "Needed Placed Dates" with a filter of "Letterbox Hybrid" would show Letterbox Hybrid caches hidden in April - December.