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It's been along time since FBAlpha core is no longer available in Retroarch, please check and addapt DIG to use the new ones.
Various FB Alpha cores are available in Retroarch and work fine with Dig.
Just edit the config for the missing core with the details of the new core
Maybe this is due to the 32/64 bits versions problem ? I had to manually switch back to 32 bits Retroarch on my 64 bit device as there are a lot less working 64 bits core in Retroarch than 32 bits.
I'm using 32 bit Retroarch. It does look like there needs to be some work done to FBA however, as the OP is actually correct that the newer FB Neo core is not in the options for selection and the FB Alpha 2012 Neo Geo core option doesn't work.