Hey @Gazza40
Just to be sure, below is the option you have enabled, right?
Settings > Biometrics and security > Fingerprints > Show icon when screen is off > On Always on Display
And AOD is set to be always visible, correct?
If both are set as described above, then please submit an error report using Beta Feedback option in Samsung Members, becuase with those enabled, you should always see fingerprint icon on AOD 
Hi this is now fixed in today's Beta release
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