For the Squash and Merge action, referenced here the PR title isn't used for the commit message; it's actually using the commit message of the single commit itself. If you add the text, "Closes issue #" back in the title before you click the Squash and Merge button, it will close the issue.

So you'll need to either add the 'closes #43' back to the title, or have it in the description for it to close out a connected issue.

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