Having trouble searching for imported video in Photos

Have you tried using a Smart Album? You can specify the keyword, there, and maybe get better results. Try the caption and text, too, just in case you put your keyword in the wrong field. If that doesn't work, search for videos, and you can restrict the date range. You may be able to get a small enough group that you can sort through them.

If you used a unique keyword, you could check in Window>Keyword Manager (⌘ k) to see if your keyword has been assigned.

Under Media Types click on the Videos smart folder. Look there to see if you can recognize the video.

Perhaps you made a typo when you typed the keyword or wrote the keyword to a different field, not the keyword field.

Which system version is installed on your Mac? It will depend on the version of Photos, if you can search for a part of the keyword or not. So, to use it in a search, type the full keyword.

And you may need to force Photos to update the library and index the new keyword. Just restart Photos, then a search should be able to find the keyword.