This looks like a weird bug:
I looked for php-errors an console-errors, but: nothing...
I decided to look at the source of the browser.
The text-area gets inline-styling: visibility: hidden;
That isn't supposed to be happening. when i remove the line, i get the complete html, like it should.
It seems that the tab-plugin has something to do with this.
https://pasteboard.co/LXPbis5HchUX.png