"My bet is that a string generated with plain "\n" behaves normal."

I went back and did an additional check on the data I used for my earlier tests. Each record in that test ends with just LF. For a specific test, I created two tiny files: check_lf which just contains "qwerty<LF>"; and check_crlf in which I forced a CRLF ending, its contents are "qwerty<CR><LF>".

$ for i in test_data test_data_Q check_lf check_crlf; do head -1 $i | 
+cat -vet; done
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX$
QueryXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX$
qwerty$
qwerty^M$
[download]

For anyone unfamiliar with "cat -vet", a newline is shown as a "$" and a carriage return is shown as "^M".

Edit: I changed several instances of NL to LF. This was for consistency with other parts of my post as well as LF being a generally recognised de facto standard (CRLF is more usual than CRNL).