Fiio m3 offially support 64gb and 4000 files in tag browsing. With 128gb it can works sort of in folder mode but it's possible you have glitches.
I mean it's a Sandisk Ultra 128GB card, one of the brand leaders when it comes to memory and storage. There isn't a device in the world it won't work with as long as it has an SD or microSD slot. There really isn't any reason I can see why it's not being recognised other than hardware limitations and the player literally not being capable of working with cards greater than 64gb in storage, in which case it's of no real use to me.