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Have you tried changing the type of the field? I notice that its a V_String, and I'm not sure if this would apply here, but I've had some weird conversion errors result in NULL values in cases where the type of the new field doesn't play well (match, or easily coerce types) with the one I'm calling using the expression.
Since you haven't obtained any sample data to work with, I'm unable to try and work out a solution. If you could upload a sample dataset I could try and diagnose this more.
If you already are familiar, then I would look at Select (11) and see what the data types are for [Task On] and [Reqs Date]. On my screen [Task On] is a date and [Reqs date] is a Double. I would use a DateTime tool from the Parse menu to change [Reqs Date] to a date and then change the formulas in Formula (30) to account for this change.
Basically Alteryx is being asked to run formulas designed for dates on fields that it isn't sure are dates. Once the conversion error limit is reached it stops trying and returns Nulls.