It seems that posts on Lemmy cannot exceed 10000 characters. If you type in more than 10000 characters and try to submit the post, you will see that the post submission is being processed but you won’t come any further than that. The post submission will remain in status pending indefinitely.
If you try to edit a post and the edited post exceeds 10000 characters then the edited post will remain in status “pending submission”. There should be no impact on the original post though. Everyone else will still see the unedited version of the post even after trying to submit the edited version.