...because hardly anyone had noticed the problem, and probably still wouldn't. Although I wouldn't call it an edge case, there do need to be a certain set of criteria met to see the problem. Stars need to be somewhat aligned, and the Swiss cheese doesn't need too many holes.
In some ways, Rigol are a victim of their own success, had they not had such a successful product, it probably would never have seen the light of day.
I do agree though, the operation of those AD PLLs is such that you do need to take care on the choice of the loop filter parts. They have plenty of documentation and tools to help the engineer with that, if the alleged parts chosen are indeed "wrong", then something should've gone a bit wrong during unit testing, when checking the PLL lock range. As I remember, there's a lock bit available on those devices as part of SPI register set, or maybe they're ignoring that, using it as a write-only part.