Not sure what the issue could be off the bat. What NuGet version is installed on your CMS site? Does the xhtml property get rendered as longstring on OPE (on-page edit) or forms edit? Or both?
No addons on the site. Xhtml property render as long string only in dynamic properties. Page's xhtml property are fine while rendering.
I have a dynamic prperty of type xhtml. When I see that property in edit mode it shows as longstring instead of xhtml in appearence.