At work, we recently updated from a massively old version of git (1.7.10) to 2.18. There are a few code bases that use subtrees, and they seem to have completely broke when trying to merge in updates.
I have confirmed that it works correctly in 1.7.10. The 2.18 behavior is clearly incorrect.
git init
echo init > test
git add test
git commit -m init
git fetch tig
git merge -s ours --no-commit --allow-unrelated-histories tig-2.3.0
git read-tree --prefix=src/ -u tig-2.3.0
git commit -m "Get upstream tig-2.3.0"
# Notice how the history are merged, and that the source from the upstream repo is in src
echo update > test
git commit -a -m "test"
git merge -s subtree tig-2.4.0
# Boom, in 2.18 instead of merging into the subtree, it just deletes everything in the repository, which is clearly the wrong behavior.