I already picked the starter in the game so the pokemon has the correct TID/SID.
Say the PID of the non-shiny is ABCDEFGH, which is valid because I picked the pokemon in-game. On Pkhex when I edit the PID to CB4ACB05, it becomes valid for the non-shiny starter. But, when I click on the shiny button it re-rolls and becomes invalid. However, according to pokefinder that PID should be shiny.