As already stated here https://answers.atlassian.com/questions/38744736/how-to-change-auto-push-to-use-tracked-branch-instead-of-origin I see the following problem:

I use a repository with two origins and multiple branches like this:

origins:
master: github
local: bitbucket server

branches (+tracking):
master - origin / master
local  - local / master

When I commit my changes I get the option to automatically push my changes.
The push target however is always using the origin remote, not the one that is tracked.

I don't see a setting to change this and this behavior leads to false commits if you forget to uncheck that box. As of this I assume this is a bug.