The builder uses Rest API to create/edit the field group (post type). It is possible that there is a problem with the Rest API on your site. You can go to Tools > Site Health and check the Rest API status on your site.
Refer to this article to know how to fix it https://meowapps.com/fix-wordpress-rest-api/
When I duplicate to Friday it does not save.
And after that the Post type is reseting to default. I change it to the Post Type I created and it does not save anymore. (see image bellow)
In fact there is a strange bug.
I removed the last group and then I was able to Select the post type again.
If I duplicate or try to create any more field it disappears and post type go do default.
Looks like a limitation.
I checked my server and there is no lack of space, memory and disk whatsoever.
I'm using PHP7.4 but also tried with 8.0 and nothing.
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