This has been known about since before the flarum based forum ended beta test, but seems to have changed recently.
Yes, the screen gives you the impression that you can add a reaction to your own post, but it has been stated that a self-set reaction will not count towards payback.
What's changed recently is that I'm sure self-set reactions never used to be saved -- ie, nobody else would see them and when you refresh the screen those reactions would disappear again.
I confirm that currently self-set reactions now seem to be sticking even after screen refresh. Even more surprising, I don't seem to be able to remove the one I set for testing.