I just tried the following: Since the Tag-Editor does not show the abovementioned fields, especially the "Composer"-field, I went to a audio-album and turned the composer in the column on; in the audio-albums, the composer is correctly shown. Also it is possible to search by the shown composer.
No, I entered the composer in this shown Composer-column also in the video-albums. And now the surprise: No search is possible! Also, during entering the composer names, MC suggests only the composers, which are entered before regarding the video files - before entering the first time in a video file, there were no suggestions - despite the fact that I had several thousands classical audio albums before tagged correctly (also the composer of course)!
For me, it seems that MC has two completely different sections for tagging and managing audio and video files - notwithstanding the Tag-Editor is the same for both.
So, please be so kind and make at least the search available for both, not only for audio but also for the now manually tagged video-fields (like Composer) which are not shown in the Tag-Editor. I emphasize once more that this would be a huge advantage over Roon (and that's what we JRiver fans want)!