This question describes the problem: https://answers.atlassian.com/questions/35771513/as-a-user-when-i-open-the-create-issue-pop-up-i-want-the-project-to-be-set-to-the-project-im-already-in-by-default

In short, I think that opening multiple tabs into multiple different projects should not change the default project you create your issues in. Instead we should store the default project client side when the page is loaded and use that in the issue create dialog.