GNU sort command does not sort words of different lengths with common prefixes correctly when using field delimiter

The GNU sort command is not sorting words of different lengths with common prefixes correctly for me, but only when using a field delimiter to sort on one of multiple fields. Here is the correct, expected sort behavior without using field delimiters:

$ cat /tmp/test0
b
c
ant
a
bcd
bc
cn

$ sort /tmp/test0
a
ant
b
bc
bcd
c
cn

Note that, for all words with a common string prefix, the shorter word sorts before the longer word. E.g. a is before ant, b is before bc is before bcd, etc. This is the accepted, standard way that English strings are sorted, e.g. in a dictionary. However, this sorting behavior changes when you are attempting to sort tabular data (such as a CSV file), and sorting on one of the columns. Here's what that looks like:

$ cat /tmp/test1
b,foo
c,bar
ant,baz
a,foo
bcd,ty
bc,pe
cn,cn

$ sort /tmp/test1 -t, -k1
a,foo
ant,baz
bcd,ty
bc,pe
b,foo
c,bar
cn,cn

Note that the words with a common prefix of a and c are still being handled correctly, but strings with a common prefix of b are not; bcd sorts before bc sorts before b, all of which is incorrect! This behavior is stable; you always get the same result. I'm experiencing this exact same issue on a much larger CSV file and the sorting errors there are deterministically random, if that makes sense. I've tried various flags to sort and none work to correct this behavior. -d and -s don't work. This is on GNU coreutils 9.4 sort for what it's worth. So, is this just a bug with the sort command? Am I somehow using it incorrectly? Is there anything better I can do that will dictionary sort the CVS by words in the first column?