First, I wanted to say thank you for taking the time to answer. Then I wanted to say how weird this is. I have three published courses that use gifs in the thumbnail. The gifs work properly.
I created a workroom - sandbox, and I imported the content from one of the above mentioned courses into this new course. The new course showed the thumbnail with the corresponding gif. The gif was moving. I decided to change the gif to a new one. The new gif did not move. To make sure the problem wasn't the gif, I replaced the new gif with the original gif. This time, the gif (which was working perfectly well when imported) did not work. It is a still image.
I decided to keep on trying. I opened a different course and replaced the still image in the thumbnail with a gif. The gif didn't work.
I am really puzzled. I do not understand why this is going on. It is very odd.
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