Steps to reproduce:

1. Open an image-containing page in VisualEditor on a non-English Wikipedia (e.g., Afrikaans).
2. Open a page on any Wikipedia that doesn't use that language (e.g., English).
3. Copy the image from the non-English Wikipedia.
4. Paste it into the English Wikipedia page.
5. Everything looks fine here. Save the page.
6. See the red mess where your image was supposed to be.
7. Open the wikitext editor and fix the File: namespace.

Example: https://en.wikipedia.org/w/index.php?diff=699995715&oldid=699995287