What version of ACF are you using? I’m assuming 5 because the wysiwyg field in 4 does not have the visual/text tabs.
What other plugins do you have installed? I’ve tested in out and I’m not seeing the behavior you describe. You need to deactivate other plugins, especially anything that modifies the behavior of the editors. You may also need to try switching themes to one of the default 20XX themes to see if that clears it up.
It’ll be easier to figure out what to do one you know where the incompatibility is.
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