Smart Lock unreliable/not working with NFC tag?

I’ve been trying to get Smart Lock to work on my Android device (OnePlus One on Cyanogen OS 13.1.2, i. e. Android 6.0.1/“Marshmallow”).
I’ve configured an NFC tag as a trusted device and want to use this NFC tag to unlock my phone instead of having to type my long password every time.

The problem is that unlocking using the NFC tag is really unreliable. Most of the time, when I activate my NFC tag on the lock screen, nothing happens, even though the pulsing circle around the lock icon at the bottom of the screen is present (indicating that Smart Lock is active).
Sometimes, however, the NFC tag is recognized and unlocks the phone without problems.
I haven’t really figured out the circumstances under which it happens.
For example, when I turn NFC off and then on again in the settings and lock the phone then, unlocking using the NFC tag once always works without problems, but the next time, it won’t work again.

What could be the issue here? Am I somehow using Smart Lock incorrectly? Does Android perhaps turn off NFC or the process responsible for Smart Lock after a while when the phone is locked (Doze mode)? If so, how do I prevent this?

Note: Also tried on someone else’s phone running Android 7 (“Nougat”), same thing: after setup, unlocking the phone using NFC works exactly once, but immediately locking the phone again and then trying to unlock doesn’t work.