That's not a sync error: it's just a demonstration that all collapse theories are inconsistent with some other assumptions we like to make. (There are many alternatives, the most famous of which is probably Hugh Everett III's relative state model, though none of them are completely elegant.) It hasn't just been discovered: it's been known since the 60s, and developed on-and-off since.
Of course, we haven't really tested this because we haven't attempted to put humans “in a superposition”. Physicalists assume that photons are adequate substitutes for humans, in the thought experiment, but something's wrong with our intuitions, so imo we should adopt some philosophical rigour about this whole thing.
Generally 'discovery' relates to experimental results and not theory.
For example, it seems like it would be more appropriate to say that the quantized nature of light was discovered in the early 20th century when confirmed by experiment and not that it was discovered in antiquity when Epicurus or Lucretius were talking about how light was made up of the smallest possible parts moving very quickly.
Also, given the parameters of the experiment in question, it arguably would be better described as "multi-layered Bell" than "Wigner's friend."
Everett's also fares well for the Frauchiger-Renner paradox too.
Of course, we haven't really tested this because we haven't attempted to put humans “in a superposition”. Physicalists assume that photons are adequate substitutes for humans, in the thought experiment, but something's wrong with our intuitions, so imo we should adopt some philosophical rigour about this whole thing.