For some reason, if I do this (on a Linux kernel repository):

$ stg new test1
$ vi Makefile --> some editing
$ stg refresh
$ stg new test2
$ vi Makefile --> some more editing, potentially on the same line
$ stg refresh
$ stg pop -n2
$ stg push test2
error: git diff-tree:

The changes don't need to conflict. I think if it requires a three-way merge it fails to push but doesn't say anything more about how git diff-tree failed.

Additional info:

$ stg --version
Stacked Git 2.0.0-alpha.1
Copyright (C) 2005-2022 StGit authors
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
SPDX-License-Identifier: GPL-2.0-only
git version 2.30.2