First of all: the problem I describe applies only to the shortcode version of your code. Inserting shortcodes in posts under WordPress theme Twenty Twenty-Two works fine, take the buttons to the lower right corner perfectly.
Same thing under Twenty Twenty: the buttons show centered on the lower edge of the screen (on Firefox, Chrome, Opera under Windows 10) and float in the lower left corner on Firefox & Chrome under Adroid on smartphones.
As I said, this happens only with shortcode buttons. If I use buttons on all webpages, they always show correctly in the lower right corner.
(I’m sorry not to be able to send you a link with the problem. I had to find a solution fast. So I skipped the button by display: none on all webpages I didn’t like the buttons to show up (dynamic CSS). This is a fast but forced solution, that I do not like. So maybe you can find the cause for the display problem and solve it. The shortcode option is a real USP, comparing your plugin to others.)