Not reproducable in a test installation. Button seems to center alright.
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The chances are high that there is some CSS or HTML which causes the html to be broken, and centering doesnt work.
Checking your page with W3 validator may show pointers.
https://validator.w3.org/
I have many errors but literally everything works except this plugin. Centering for everything else works. Ads, text, images, etc.
There is no certainty where a broken html or css rule will distort the html. The html and css used with the locked button are valid, as you can observe.
Therefore there is nothing that can be done on the plugin side to address this issue. Maybe you can try to !important the css for the button, but then again this likely would not work since broken html cannot be fixed that way.
Broken site html is not something that should be left alone in any case.
