Hi, i have this problem too,but i could not solve it even following what you did,
I tell you what I did so maybe you understand where I went wrong … I created a Media Player object and like you it works only in editor if I select the correct stream option.
I created a material with and entered the mediaplayer information into a texture sample and linked to the emissive color.
then i opened the common blueprint layer and i opened 'url of the webcam and referencing a variable mediaplayer
everything works in the editor but in runtime it seems that it cannot modify the playback options of the media player. what am i doing wrong in your opinion? any help is welcome