I am having the same exact issue and was going to create a topic but saw this one first.
I disabled all plugins except GP Premium. No problem and I can toggle between Text and Visual when using Sections.
However, when I enable the Classic Editor, the problem happens. Gets stuck on the Text tab and cannot change to Visual. When clicking Visual, it adds that HTML code as specified at the top of this post.
GP version 2.3.2 and GP Premium plugin version 1.8.3.