Yep, your text is too long. Below the textarea (left corner) is a counter which shows you how much chars you used and how much the maximum is. Here the max is 5000.
I'm surprised it let you submit the post and not show an error with an proper message.

I can see why it troubles you. But on the other hands admins and devs need to set limits to prevent bad actors from using up the server resources with just a few requests. I'm sure that there are other Fediverse platforms which allow much larger text, so these might be something for you (or your host your own as your said).