I already contacted the support. They replicated the issue and DEV told me:
Interesting issue. It looks to me that the duplicator plugins are causing the post_content data to become corrupt in some way when the post is copied.
This may not be a specific issue in the duplicator plugins themselves, but may be an issue with one of the WP post_content filters being run when the post is inserted into the DB.
I’ll add this to my list of issues to fix, but won’t be able to provide you with an ETA on a fix just yet.
It would be great if you could perform some more investigation to determine if there are some use-cases when the duplicator plugins do work.
For example, create a new block type with a very simple field group (one text field). Try to replicate the issue on a new post, adding only one of this block type. If this works, try adding two blocks. If this works, try adding more complex field types to the block, etc
I plan to try some investigation this weekend and anyone else can too 🙂
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