Imported Data Table does not match Data Transformation result

[uhdacarter]

I am having a very strange problem with an Imported Data Table not matching the Power Query results.

I am building a table of 10 dates which are the distinct date values occurring in a "Pull Dates" column of a much larger table.
I need to label those 10 distinct dates with sensible descriptive names that include the date itself for reporting slicing.

The Transformation is working as expected, as the results of the final step are what I want and expect.

When I "Close and Apply", and then go to the "Data" tab and select that imported table, the table is slightly wrong - different from the trasformation results.
Transformation Results: 10/25/2021 has the correct label (enclosed in green)

Imported Data Table: 10/25/2021 has the wrong label (enclosed in red)

The M code from the Advanced Editor is here:

let
Source = List.Sort(List.Distinct(Table.Column(CombinedData,"Pull Date")), Order.Ascending),
// 
SourceLength = List.Count(Source),
ReportDate = List.Last(Source),
// 
Back1List = List.FirstN(Source, SourceLength-1),
Back1Length = List.Count(Back1List),
PreviousDate = List.Last(Back1List),
// 
Back2List = List.FirstN(Back1List, Back1Length-1),
Back2Length = List.Count(Back2List),
TwoReportsBack = List.Last(Back2List),
// Calculate "ReportDate" one year ago.
OneYearAgo = Date.AddYears(ReportDate,-1),
TwoYearsAgo = Date.AddYears(ReportDate,-2),
ThreeYearsAgo = Date.AddYears(ReportDate,-3),
OneYearEnrollMilestone = "",
OneYear1stClassMilestone = "",
TwoYear1stClassMilestone = "",
ThreeYear1stClassMilestone = "",
// 
DateTable = Table.FromList(Source, Splitter.SplitByNothing(), {"Pull Date"}, null, ExtraValues.Error),
#"Changed Type" = Table.TransformColumnTypes(DateTable,{{"Pull Date", type date}}),
AddDateLabelColumn = Table.AddColumn(#"Changed Type", "Data Date",
each 
if [Pull Date] = ReportDate then "Report Date" & " " & Date.ToText([Pull Date])
else if [Pull Date] = PreviousDate then "1 Report Back" & " " & Date.ToText([Pull Date])
else if [Pull Date] = TwoReportsBack then "2 Reports Back" & " " & Date.ToText([Pull Date])
else if [Pull Date] = OneYearAgo then "1 Year Ago" & " " & Date.ToText([Pull Date])
else if [Pull Date] = TwoYearsAgo then "2 Years Ago" & " " & Date.ToText([Pull Date])
else if [Pull Date] = ThreeYearsAgo then "3 Years Ago" & " " & Date.ToText([Pull Date])
else if [Pull Date] = OneYearEnrollMilestone then "1 Yr Rel. Enroll Open" & " " & Date.ToText([Pull Date])
else if [Pull Date] = OneYear1stClassMilestone then "1 Yr Rel. 1st Day" & " " & Date.ToText([Pull Date])
else if [Pull Date] = TwoYear1stClassMilestone then "2 Yr Rel. 1st Day" & " " & Date.ToText([Pull Date])
else if [Pull Date] = ThreeYear1stClassMilestone then "3 Yr Rel. 1st Day" & " " & Date.ToText([Pull Date])
else "Date Label Unknown" & " " & Date.ToText([Pull Date]))
in
AddDateLabelColumn

Any clues why the two are different?

[KNP · Accepted solution]

It's a bit of a stab in the dark but can you try adding a List.Buffer() to the source step?

Source = List.Buffer(List.Sort(List.Distinct(Table.Column(CombinedData,"Pull Date")), Order.Ascending)),

[uhdacarter]

Hello KNP,

That seems to have solved the problem. Now I wish the M Documentation had a more in-depth discussion of what List.Buffer does and when it is appropriate to use it.
Since you brought this useful little function to my attention, I have read some commentary on how inappropriate use of List.Buffer can adversely affect performance, but so far haven't had time to read more.

Thank you, again!

[KNP]

Hi @uhdacarter,

Yes, List.Buffer and Table.Buffer can be a bit of a mine field. 

They can, and have for me, improved performance significantly in some cases but can certainly have the reverse effect also. I believe it is a guaranteed way to break query folding if you have a source that folds, but don't quote me on that.

If List.Buffer became a problem in your scenario, I guess you add the List.Sort function everywhere the source was mentioned although I'm not sure that would be efficient either.