I am trying to impose a brand standard intro screen (thumbnail) on my clips. To do this I have created a short mp4 video and added it as an intro in the clip, saved the changes and published it to YT. The published clip shows a thumbnail that is neither my intro nor the first image in the clip. I originally had a 01.s intro video. I changed this to 0.5s, to no avail.

I have separately been manually editing the title generated by Opus and wanted to complement this with a visual element that reflects my brand.

Visual differentiation of shorts seems to be key, some of your rankings actually suggest adding a visual element. How can I do this, so that it acts as a short thumbnail? Is there a workaround?