Steps to reproduce:

• Create new page
• Paste an image into the editor
• Watch the upload dialog saving the image as attachment
• Press Cmd+Z (MacOS) / Ctrl+Z (Windows or Linux)

Expected result:

• Image is removed

Actually happens:

• A block of text with %MCEPASTEBIN% appears in the document and another Cmd+Z/Ctrl+Z is needed to remove it

Notes:

• If saved, this block of text is not stored in the page
• Affects Chrome 79, Firefox 72, Edge 80 (Chromium), Edge 18. In Chrome, the block of text is very narrow and appears to be written vertically. Firefox shows it as regular text.
• Only reproducible when collaborative editing is enabled. It does not affect comments.