I had a very similar problem with my EZ Flash 4. I think the internal battery died on the chip (before I even got it), so the save data was not being retained, yet the cartridge would still try to save when it powered up - and basically it would write a blank save and delete all my progress.
I...
although i ended up accidentally changing my nand with no way of going back bc i didn't know you needed a gamecube controller to restore it (i have a wii minus/1.1, which doesn't have gamecube controller support)
and if i find the sd card i have my NAND backed up on, i can not only restore the NAND if i solder the right part in, but i can also put my NAND on dolphin, then i can play dolphin online, like PMEX Remix or MKWii Riibalanced