Action not showing up as either NEXT or AVAILABLE
 

I have a single action under a project (parallel). It has a start date of today and is due tomorrow. When I filter with availability of either NEXT or AVAILABLE, it doesn't show up. It does show when I filter availability as REMAINING.

This makes no sense to me. If next and available are the subsets of remaining, than how can it show as Remaining, but not as next or available. Also, surely it is either next or available, right?

Any help in understanding what is going on would be very appreciated.

What time is it starting today? If the start date is still in the future, that's exactly the behavior you should expect. If the project has a start date that is still in the future, you'll also get the behavior you described.

Next actions are a subset of Available actions. Available actions are a subset of Remaining actions. A Next action is an Available action which is the first uncompleted action in the project, reading from the top down in project view.