Thank you for your question.
We have tested the scenario and can confirm that the described behavior is observed. However, this is not an Editor's bug, but rather browser-specific behavior. You can confirm this yourself by creating a sample web page and placing a video in an editable iframe. For your convenience I have attached one such example.
You can easily observe that exactly the same scenario is replicated in the plain iframe element. Unfortunately, I cannot provide any suitable workaround for the moment.
I hope that the provided information was helpful. Feel free to contact us if you have any other questions.
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