I asked an LLM and it described the problem as "covariant typing of mutable collections" or "unsound covariance". I'm not mathematically educated in this area but that sounds right?
Yes, that's correct. If you're curious for a slightly more concrete source, Wikipedia covers this reasonably well (although citing one often decried source to validate another might not be particularly convincing to some people): https://en.wikipedia.org/wiki/Type_variance#Arrays