So I've created a dashboard and published it to Tableau public, but there are three different situations and they all look different. The dashboard is fixed so it should be fine, below are images of the three situations.
Tableau Public(When I download Image)
Tableau Public (What I see on my screen when I publish it)
Which font are you using? It looks like it might not be a websafe font, which would cause it to render differently on Tableau Public: