Step to reproduce:

Import this CSV once (TAB):

_id x
5ffef2488fef3e58320867d0 1
5ffef2488fef3e58320867d1 2

Then change it like this:

_id x
5ffef2488fef3e58320867d0 2
5ffef2488fef3e58320867d2 3

And try the import again.

The import has no effect. I would expect either an error or that the first document would update and the second would be inserted.