Bit of a PSA. You might not want to trigger the previous generation Legendary encounters in the postgame, because Gamefreak screwed up coding the IVs. All the Legendaries are SUPPOSED to have 3 perfect IVs, but they ended up with random IVs.
According to Pokemon Scarlet and Violet players, the Legendary Pokemon encounters in The Indigo Disk are affected by a noticeable issue.
gamerant.com
Gamefreak will probably end up patching this eventually, but who knows when. I'm not sure if I actually care or not, but it's just worth knowing before getting too far in.
All the past Legendaries are also Shiny locked, but that was probably on purpose (BOO!).
Although, on the bright side, this does seem to be the only big bug in this DLC. So, that's a nice surprise!