I to can replicate this error. https://youtu.be/8AzpmBXuu4U hope this helps, this is while in a development store with generated content for developing an app ( theme extension only.
Mines on a development store too, but one that is being transferred to the client soon (so no early access features).
Seems to happen regardless of browser, I copied and pasted the content from Microsoft Word - even editing after saving the issue was still present. I checked the .json file and there wasn’t anything amiss with the characters either.