I am having this issue too. It seems to only affect me when I’m using an ACF php block and have the latest Gutenberg editor plugin installed. It does not do this with stock gutenberg (meaning no plugin).
“Also, when both ACF PRO and Gutenberg 6.4 are active, my ACF Block with a wysiwyg field in it does not seem to initialize the tinymce editor easily when I click into it.”
Besides the warning message I showed in my screencast, I also get the tinymce initialization issue, BUT only when going from preview to edit mode. Clicking the “Visual” or “Text” tabs forces TinyMCE to wake up and come to life !