There’s honestly not much I can tell you there. WordPress just strips (or a better phrase “tries to convert”) any code you’re manually putting in for the visual editor. The issue doesn’t have anything to do with the theme. Try posting on wordpress.org support forums for possibly more advice other than obviously just not using the Visual tab.
This is searchable archive of our old support forums, which operated from 2012 - 2016. To find out how to get support for your current theme, please visit our support page.