Wrong value is returned when calling a function from a cell - vba

Question

First post yall.
Long story short, in Excel, when I call the following function (it's in its own module) from a cell, it returns the wrong value. The function returns the correct value when calling it from a sub, as well as when I step through the code (to the end), but the moment I call it from Excel, it returns a different value. Background at the bottom.
Things I've Tried
Making it a Public Function
Giving it an argument
Changing the function and/or module name
Moving it out of a module
Restarting Excel
A bunch of random stuff
It really is just this specific function that's giving me this issue, simpler functions do what they're told. I have to assume it has something to do with the order of events Excel is doing things, or the limits of what parts of Excel a function can change.
Function ActiveDisciplineFilters()
Application.Volatile 'makes the function update automatically
Dim disccolumn As Range
Dim uniquedisc() As String
Dim uniquediscstring As String
'create a string of unique values from the Discipline column
i = 0
If Range("LayerList[Discipline]").SpecialCells(xlCellTypeVisible).Address = Range("LayerList[Discipline]").Address Then
ActiveDisciplineFilters = "None"
Exit Function
End If
For Each cell In Range("LayerList[Discipline]").SpecialCells(xlCellTypeVisible)
If InStr(1, uniquediscstring, cell.Value) = 0 Then
If i <> 0 Then
uniquediscstring = uniquediscstring & ", " & cell.Value
Else
uniquediscstring = cell.Value
i = 1
End If
End If
Next
ActiveDisciplineFilters = uniquediscstring
End Function
Background
In Excel, I have a table. I'm taking all the data in one specific column of that table and creating a string of the unique values in that range (separated by comma). That string must be placed in another cell, for reasons I don't need to get into. If a filter is applied to the column, the unique values update automatically.
What would make Excel give me the right answer when I call it from a sub, then the wrong one when I call it from a cell?

Answer 1

Unfortunately, none of the SpecialCells methods work in a UDF. If you need this to be run from the worksheet as a formula, then your code should look like this instead:
Function ActiveDisciplineFilters()
Application.Volatile 'makes the function update automatically
Dim disccolumn As Range
Dim uniquedisc() As String
Dim uniquediscstring As String
Dim i As Long
Dim cell As Range
Dim bHidden As Boolean
'create a string of unique values from the Discipline column
i = 0
For Each cell In Range("LayerList[Discipline]").Cells
If cell.EntireRow.Hidden = False Then
If InStr(1, uniquediscstring, cell.Value) = 0 Then
If i <> 0 Then
uniquediscstring = uniquediscstring & ", " & cell.Value
Else
uniquediscstring = cell.Value
i = 1
End If
End If
Else
bHidden = True
End If
Next
If Not bHidden Then uniquediscstring = "None"
ActiveDisciplineFilters = uniquediscstring
End Function