It was definitely an issues with the dates. After switching fields with the min function it worked so I assume the min function does not work with dates? Either way I tried a different solution to find the earliest date and OrderBy fit the bill.
First attempt:
var filt_val = $feature.Field1
var fs = FeatureSetByName($datastore, 'combined_points')
var list = Filter(fs, 'Field1 = @filt_val')
var OriginalEntry = First(Top(OrderBy(list, 'Field6 ASC'), 2))
var OriginalNorthing = First(Filter(list, 'Field6 = OriginalEntry.Field6'))
return $feature.Field2 - OriginalNorthing.Field2
This was valid however it wasn't the solution I expected as I was not getting the value from the earliest date. So either a problem with line 5 (my sorting) or 7 (the filter using the earliest date). I found out it wasn't my sorting, see below. Not sure what is causing the filtering from line 7 to not work as expected, maybe another day I will figure that one out.
After some trial and error this is working so far, will test some to make sure:
var filt_val = $feature.Field1
var fs = FeatureSetByName($datastore, 'combined_points')
var list = Filter(fs, 'Field1 = @filt_val')
var OriginalEntry = First(Top(OrderBy(list, 'Field6 ASC'), 2))
var OriginalNorthing = OriginalEntry.Field2
return $feature.Field2 - OriginalNorthing
Thank you again for all of the help. I will have to do some more research on the AGOL / Portal expression builder for the future. I am used to fumbling through python notebooks in Pro and being able to print intermediate results is a must!