TinyMCE: fib gaps not visible

How to reproduce:

• in the question editor for gap text add a video
• below the video add two gaps and save
• in the score tab choose "score per answer" and save
• NOK:
  • in the preview / preview solution / test run the gaps are not visible
  • after reopening the gap text editor the gaps are not displayed there, too
• Note: in the score tab all gaps are still available under "score per answer"