I think this is related to the bundling of widgets to a single file.
The Audio Video Playback widget uses just such a minified file. Probably the JWPlayer does so too..
You can direct Mendix to bundle the widget files when running locally too, through Settings > Modeler.
For me, enabling this results in the same problem you describe when running in the cloud (tried it for Audio Video Player).
I'd suggest filing an issue with the widget developer on GitHub here.
Alternatively, if you're comfortable with tinkering with the widget yourself, you can swap out the minified version of the libraries with unminified ones. For me this solved the issue (again, tried it with Audio Video Player).
Or you can grab my fork of the widget and try that.
Hope this helps!
I checked and Jeroen is correct on this one, it has to do with widget bundling. I have done my fair share of widget building and found the following problem:
So the libraries that are in Audio Video Playback (didn't check the other one at the moment) have a UMD wrapper, which should work fine. The problem? Because it is minified, it doesn't get picked up by Dojo Build. It's a 'dumb' problem basically. Found this to be the issue while building a Typescript Widget Boilerplate, that you can find here (this is not production ready yet, but does seem to work).
Still trying to find the exact premises on when this issue occurs. My minified code is wrapped as AMD, not UMD, which works fine.