Steps to replicate the issue (include links if applicable):

• Edit any Page on WS.
  • E.g.: https://pl.wikisource.org/w/index.php?title=Strona:PL_Boniecki_-_Herbarz_Polski_(1).djvu/10&action=edit
  • E.g.: https://en.wikisource.org/w/index.php?title=Page:Grimm-Rackham.djvu/27&action=edit
• Don't change anything.
• Press the Preview button.

What happens?:

The preview shows changes in:

• Page status. Note that even when status is "Not proofread" it shows you changed status to "Not proofread".
• Removed header.
• Removed footer.

What should have happened instead?:

Only changed fields should be shown. Same as in standard preview.

Software version (skip for WMF-hosted wikis like Wikipedia): pl.ws, en.ws.

Other information (browser name/version, screenshots, etc.):

Changes shown when no changes were actually made:

Note that preview works fine without AJAX preview (dynamic preview). The problem only occurs when you enable AJAX preview: