Thanks for the tip - I really hoped it might solve the issue, but unfortunately it does not.

For the file test.txt (TAB separated fields) being

a_2     2
a2      1
[download]

I still get

$ sort -k 1,1 test.txt | cut -f 1
a2
a_2
[download]

whereas when there is no second field then the result is reversed,

a_2
a2
[download]

Looks like the problem 'survives'.
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