I was about to create a new topic when I saw this. I was experiencing the same issue after a clean install with the default theme and just meta-box and mb-custom-fields installed (PHP 7.4.1)
when clicking on custom fields add new the editor shows up then disappears:
https://recordit.co/IuxSI2I4zq
The issue appears to be triggered if the permalinks are set to plain. Setting permalinks to post name seems to fix it. If permalinks are set to plain and this is not supported a warning would probably help a lot of people out who just installed meta box on a clean install otherwise they might just think the react version of mb is broken.
