This is a bug typically met if grouping on a memo field.
There may be several workarounds depending on your needs:
Select
a, Left(b, 255) As b
From
table1
Group By
a, Left(b, 255)
Select
a, Mid(b, 1) As b
From
table1
Group By
a, Mid(b, 1)
Select
a, First(b) As firstb
From
table1
Group By
a
Select
a, DLookUp("b","table1","Id = " & [table1]![Id] & "") AS b
From
table1
Group By
a, DLookUp("b","table1","Id = " & [table1]![Id] & "")