So summary of the conversation:
1. Definitely bootleg cart
2. Using GBAbackup tool to dump and restore unmodified saves (at that point) works fine [save not stored in ROM]
3. When viewing save in HxD, the save appears to be mutated.
4. With further testing, it appears the game expects to load a mutated save.
5. By using PKHeX on the save first (first partition of save is fine), then copying the block at offset 0x0 to 0xDFFF and paste overwriting to 0x10000, then subsequently restoring the save, the PKHeX edited save was loaded by the repro cart. I imagine the cart reads the save starting from 0x10000 but who knows. [we tried with an unedited save but rearranged to be not-mutated, didn't load]
6. I advised that one either may not be able to beat Elite 4, or could beat them but won't have the HOF records.
In any case, the steps:
Issue is now resolved