[OP3] Fingerprint scan ignored when ambient display turned off

I noticed this weird behavior on my OP3 today when I turned off ambient display. When I unlock the phone with fingerprint, it still prompts me to enter PIN. It happened multiple times, so it's not the usual "reminder" we get once in a while.

I'm not sure if it's because I have active notifications, but it definitely doesn't do that when ambient display is enabled. Also it doesn't happen when I don't have notifications or have just locked the phone.

Has anyone else noticed this? Any idea why this is happening?

UPDATE: I think it happens when the device hasn't been active for a while. It's like it forgets the fingerprint.

UPDATE 2: Looks like this was caused by the Light Flow's device admin feature being turned on. After I disabled it, fingerprint scanner works fine.