I can't put my finger on it, but it also seems to prefer doing that for some topics than others. Some videos will have the effect you describe, while others will basically just disappear the whole topic after you watch the video. So far it feels like it prioritizes spamming you with videos on the same topic when the 'seed' video is shorter, more sensational and has some other elements of making the video more 'addictive'. Like it's trying to push you towards Shorts.