> requires a cd to the specified directory.  With the CDPATH variable set,
> cd to a relative path prints the full path of the destination to standard
> output.  That is collected as part of the standard output of the command

Ah, that explains.

> I can redirect the output of cd to /dev/null.

Yep, I do this in my scripts as well.

And for all current/future users: Set CDPATH, but don't export it[1].
That would also have avoided the problem. I did this now :/

[1] https://mywiki.wooledge.org/BashPitfalls#pf48

Thank you, Julia!