Git2.51 now seems to fetch the HEAD branch from a remote (which it doesn't with git 2.46), which now appears in Sourcetree as e.g. origin/HEAD. In some cases it's listed before the actual branch (seems random which remote names or branch names it happens to), such that when checking out or merging this branch, it will default to HEAD instead of the actual branch I want to merge or checkout

It might seem like it sorts small case branches first, then HEAD, then any branches that starts with uppercase. This causes issues for my team due to searching keywords in the merge text, and also that I never want to checkout a branch called HEAD, when there is a branch called e.g. Develop

Somewhat related is this case, where I have two branches on the same commit, and select merge, I would strongly prefer the option to select which branch name I want to use in the merge subject. The problem should be clear in this example:
trying to merge

gives

which is clearly not what I want.

Using the branch picker that is launched when you checkout this commit would help a lot: