I looked at all the datasheets I could get my hands on, and while they mention the RID functionality, they don’t explain how to configure it at all. This libfreefare GitHub lists that once you configure RID it can’t be undone =/ I’m not sure about that though. I’ll keep looking into it.
When you scan your flexDF with Taginfo, does it give you a changing UID that is only 4 bytes long, and the first byte is always 80?