I looked up the Flexar card you describe. Part of the description is 1000x (?which likely refers to CD speeds for some unknown reason?). The actual read speed advertised is not 1000mbps, but 150mbps...
Anywho... The card itself is UHS-II and should be backward compatible with the UHS-I SD slot in a Lumia 950. However, firmware might not be tuned right for this card... Just a guess.
As libra89 says, test the card in a PC to see if it is a good card.
How is it formatted? At 128GB is should be exFAT, not FAT32.
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