Video install strange behaviour

[Joned]

Web Designer Premium.
I have attached a screenshot to aid my description. I hope it is clear enough to view.
I have a website that I often install MP4 video's on webpages using a "Place Holder" rectangular graphic Box. This has worked without any issues for a long time.
Now all of a sudden when installing the video HTML code into the 'web properties' HTML (body) window I get screenshot 2. (just a dotted rectangle with the widget tab). The Black rectangle disapears.
When I deselect I'm left with an empty space - screenshot3. No indication anything is there.
The good news is that when uploaded to the web all is good and video displays and functions normally. So I guess you could argue "whats the problem then". It's not really I suppose but I'd like to know whats caused it and whether I can undo the fault.

Any comments welcome.
Thank you.