With the following two simple files:

a.txt

1 a
2 b
5 c

b.txt

2 x
4 y
5 z

The following command does not behave like expected:

$ join -a 1 -a 2 -e 0 a.txt b.txt 
1 a
2 b x
4 y
5 c z

I would expect the option -e 0 to fill up missing values with zeroes. However, the following does work:

$ join -a 1 -a 2 -e 0 -o auto a.txt b.txt 
1 a 0
2 b x
4 0 y
5 c z

Reading documentation from $ man join, I see no connection between -o and -e that would make the above behaviour meaningful. Instead, I find it misleading that a useless -o auto needs to be inserted into my command for -e 0 to work..

Is there an explanation, to be clarified in the manpage? Or is it a bug?

(btw:

$ join (GNU coreutils) 9.0
Copyright (C) 2021 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel.
)