Eevees caught in the Pokéwalker are being flagged as Illegal if they are evolved before being transferred.
I was transferring some Pokéwalker Pokémon from SoulSilver to Black, but my Leafeon was marked as illegal.
I loaded the SS save into PKHeX, and the Leafeon is considered legal in SS; it is only flagged in Gen 5 and above with, "Encounter Type PID mismatch" and, "Encounter Type does not match Encounter."
So I restored a previous version of my save that had the original Eevee before I traded it to Platinum and evolved and transferred it to Black, and it is marked as legal in both Black and SS.
If I change the Eevee to a Umbreon, Espeon, Leafeon, or Glaceon in the Black save using PKHeX and increase it's level from 10 to 11 then it is not flagged.
I have included both a pk4 and pk5 file of the Eevee and Leafeon affected and an editied Umbreon that is considered legal.
Changes made to the Umbreon:
1. Species changed from Eevee to Umbreon.
2. Level Changed from 10 to 11.
No changes were made to the other Pokémon. Just a drag and drop into a folder from the respective saves.
133 - EEVEE - 85B16900004C.pk4
133 - EEVEE - DF246900004C.pk5
197 - Umbreon - CC1D6900004C.pk5
470 - Leafeon - 7DD16900004C.pk4
470 - Leafeon - D2536900004C.pk5