AC Tents I saw on YouTube have both upper and lower light leak blocks on their tents, they cover the zippers and they meet in the middle and slightly overlap to block light. The picture you showed has a top lip, but no bottom lip, and I am wondering if that is because it is caught in your zipper which would cause it to get stuck.
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Happened to my old flower tent, but maybe our tents are different. I was stumped for hours too until I picked it out, sorry if this doesn't help I just thought I recognised something that had happened to mine.