Do you happen to know if rs1229984 is linked to any particular group that could make this result problematic? I noticed that the paper mentions that "There was also some evidence for a difference in years of education, and, while the size of the effect was small, this requires further investigation." That sounds like a decent-sized red flag.
Take a look at the population frequencies on the right. While the minor allele is infrequent in Europeans, it is extremely common in East Asians, for example.
It seems to me that we would not have to know about the link in order for the conclusions to be unfounded. These MR analyses rest on the assumption that the instrument is not linked to any potential confounders. This seems like a bold and unwarranted assumption when we are talking about an instrument linked to behavior, given the complexity of the chain of events (chemical, biological, and psychological) between gene expression and observable human behavior.
That education red flag involves just one possible confounder out of many that were never considered.