stage1: fix incorrect splitting function

[squeed]

My previous PR used the wrong splitting function 🙇

[s-urbaniak]

LGTM, but I am worried that we didn't catch this with a functional test. Do you mind to create a follow-up issue, or, even better, add a test here ;-)

[squeed]

A good question.
The reason we didn't catch this with a functional test is that the function I refactored swallowed all errors "because best effort," so I emulated that.

Probably this function should output some debugging information we can match on.