Thanks for the update! I saw it didn’t work for me, but only now have I found a time to look into it.
So, the issue is not only with <p> tag, but with any block tags (they are listed in $allblocks variable in the wpautop source). And the issue is actually the WordPress issue, not yours – including a simple
<script>
alert('<div> This is sample HTML </div>');
</script>
in the text view breaks the post.
So, there’s probably not much to be done on your part (except for the solution I outlined above, but it may break other things, I suppose). I’ll try to reach out to the WP devs.
Update: I’ve managed to fix this problem in my development environment. The extra line breaks are introduced by the wpautop filter, so removing the two lines that change its priority in wp-d3.php fixes the issue. Still, I would prefer not to modify anything myself on the actual website, so it would be really great if the bug was fixed.
