Gutenberg is an editor, it’s not an active process that runs in the background. Unless you opened the post in the block editor there is no way for it to modify the HTML
Think of it this way, a PDF is a PDF, it’s content, data. If I create it with one application, then copy it to a machine that has a different PDF reader, the PDF is still the same. To modify it I would have to open the PDF in an editor and resave it.
The same is true of Gutenberg. Unless you open a post in gutenberg and press save/publish, there are zero opportunities for Gutenberg to make modifications.
Try to imagine what would be involved for Gutenberg to actually do this. It would need to update every single post in the database, track what it had and hadn’t modified, and it couldn’t do this in a single request, it would have to continuously poll itself until the job was done, creating major server load.
So gutenberg is not the cause of your issue. It can’t be, unless you opened all 1200 posts then edited and resaved every single one of them.
So what can you do?
First, do a standard WP export, without any additional plugins interfering or meddling.
Second, check the markup in the wxr and see if it has the same content or if it’s been modified.
Third, import the data, and check the markup in the database, not the editor/frontend.
Fourth, do all of the above with the standard default themes and no plugins.
Put simply, there is no evidence Gutenberg is at fault ( there’s no evidence of what is at fault, the cause is currently unknown ), and Gutenberg doesn’t work that way.
As a final note, what you’re referring to is not normal content, and those aren’t paragraphs. Those are TinyMCE Advanced “classic paragraph” blocks, and they may not work the same way. For example, it might be that how the content looks and what the actual HTML is are not the same.
I’d suggest making sure TinyMCE Advanced is also installed on the target site. Otherwise you should ask for TinyMCE Advanced support at the TinyMCE Advanced support forum