Something can both be a historical pattern and popularized as something we pay attention to. There are also historical patterns that many people don't talk about frequently or pay attention to.
Aside from that, I'm not sure how you're so confident that there is no observer effect. Markets, particularly in the short term, are influenced by human perception and emotion. It is plausible to me that, in particular, the stock market could dip because everyone observes the yield curve inversion, gets nervous about a coming recession, and then moves money out of the market in fear of it. This could happen even if a recession does not, and I don't find it impossible that such a move could help contribute to an actual recession. Again, human perception is an enormous component of markets, and perception is influenced by emotion.
Aside from that, I'm not sure how you're so confident that there is no observer effect. Markets, particularly in the short term, are influenced by human perception and emotion. It is plausible to me that, in particular, the stock market could dip because everyone observes the yield curve inversion, gets nervous about a coming recession, and then moves money out of the market in fear of it. This could happen even if a recession does not, and I don't find it impossible that such a move could help contribute to an actual recession. Again, human perception is an enormous component of markets, and perception is influenced by emotion.