Actually this issue is not happening in our test installation .
And i have tested the same shortcode that you have used in your site and i see no issue .
Can you pls remove the button shortcode and add afresh button shortcode and see if it helps.
I have done what you said,
deleted the shortcodes for the buttons and added brand new ones.
Typed each out very carefully to make sure nothing was typed wrong.
Still having the issue.
This issue only recently started after updating wordpress and the theme.
I do have some custome CSS but they never caused an issue before. And its all custom CSS that was suggested in previous threads I have started asking for support.
I have disabled the plugins and still having the issue :/