for the same reason as
q)(a:1)
'rank
[0] (a:1)
^
at least one of the columns needs to be a vector, and the remaining scalars will be extended to the same length. e.g.
q)(a:1;b:0 1)
a b
1 0
1 1
in your scenario
not_working upsert flip (column1`column2)!(2017.12.19T11:56:02.544;1#12)