There's a bunch of missing locations for C/XD encounters due to the rebattling at another location.
It doesn't really matter after transfer to Gen4+ (location is reset to Pal Park), but we can always add more location entries to the encounters file.
https://github.com/kwsch/PKHeX/blob/0120263bd3d56603c703d91521ba62a3a1a6e61b/PKHeX.Core/Legality/Encounters/Data/Encounters3.cs#L416-L424
You can check the location ID by loading the pkm to tabs, then opening the batch editor, then selecting Met_Location from the dropdown. The location ID's value should be shown in a label above the textbox.