Tasks won't work on a standard work day

I'm recreating a program for an EOT claim. I did the basic work in MSP and now in P6 I'm adding logic etc.

The problem I've found is that even though the 17 Aug 2016 is shown as a standard work day in the calendar, one task E1100 treats it as a non-work day.

The tasks are all Task Dependent

The calendar came in from MSP and has no parent

There are no resources or resource calendars in play

There's an obvious workaround to create a new calendar, but I would really like to understand what could have gone wrong here as this could have serious consequences in an EOT claim.