They might be tricking the GBA into thinking it's a GB cartridge (it has a built-in GBC "emulator"). I found different ways this could work online: perhaps by adding extra plastic to actuate a switch (that switches between 5 V (GBC) and 3.3 V (GBA) power to the cartridge); alternatively, the ROM chip is made to withstand both voltages and the system selection is done by a 0 in the top bit of the cartridge's "B" register.
So admittedly I bought all of these off eBay having done zero research. They’re all bootleg? If so, the physical quality is very good, but it could explain why Link’s Awakening won’t keep a save.
Yes they all look bootleg to me. If you compare the sticker online you will most like find subtle differences. You can also open the case and look at the board which will tell you for sure if they are real or not.
Also Links Awaking and both Oracle games never released on GBA they are all GB/GBC games.
Links Awaking had two releases. The original Links Awaking is a GB game for the original Gameboy and Links Awaking DX is a game for the GBC with extra features and colour support.