Custom block source code dissapears / not editable

Source code of a custom block disappears when editing the custom block.

Steps to reproduce:
1. Create custom block.
2. Use "Full html" option
3. Change to "Source code".
4. Enter html code.
5. Save -> Visit Website.
6. Use "Edit button" to edit block.
7. HTML Code is visible for a second, then dissapears. No way to edit the code.

See video attached.