FLAC files getting corrupted on microSD card
This is almost certainly an SD card issue, especially since initially the files are fine and not corrupt, and get that way over time. Your device and it's applications will only read the data from the SD card, not change it or write to it anyway. There is a very slim chance that the card reader in your phone is defective, but I think a safe bet would be the card itself is failing. 

This is the most definitive answer I can offer with the information given. 

I am not normally one to recommend a specific brand here, but my personal experience with SanDisk has been very disappointing in terms of reliability in multiple product lines they carry. I would recommend moving away from SanDisk and trying perhaps a Samsung EVO+ series card.