That was it! The culprit seemed to be a rather poorly written app from our city's public transit company, that is supposed to use NFC to read transit card info, but actually never works. The keyboard issue appears to be gone with this app removed.
Thanks for the tip! I'd never have guessed it, the app wasn't active or even open as a background process.
The only place I've seen the 4-hour time limit documented is this help article from Google: https://support.google.com/android/answer/6093922?p=personal_unlocking&rd=1
My Nexus 5 always refuses to unlock with NFC Smart Lock every morning (after 5-8 hours idle). After unlocking it once by hand (by drawing the pattern), NFC unlocking works again.
The bug is a different issue - it happens at random, whether or not the device has been idle for > 4 hours, and the affected users need to toggle NFC, Smart Lock, or restart in order to have NFC unlock working again.
You'd have to convince the staff to register your ring as a valid tag, if they are compatible. Most of them are, the hard part is the convincing...
Can confirm 2016 rings work with Virdi AC2100:
http://www.virdi.eu/produkty/1,terminale_biometryczne_+_rfid/2,virdi_ac-2100.html
Just a heads up, the Tags app is still there in Android 6.0.1 Marshmallow, but to see it you have to go to Settings -> Apps, tap on the menu icon (three dots on the upper right) and select "Show system". From then on, Tags will appear on the apps list and you'll be able to disable it from there.
Also, with NXP TagWriter, you can completely format a tag to factory defaults, which is a step beyond just erasing the content. That might help to ensure it reports nothing but its ID to unlock.
I tried http://www.thesayansblog.net/2017/11/re-enable-nfc-smart-lock.html but it didn't work for me.
I have found another way to re-enable the NFC Smart Lock feature on Android, though it still requires root and manually editing a preferences file.
Details on pastebin: https://pastebin.com/2RSZzERg
@jasok2 Some NFC writers (I use the TagWriter app for Android) let you set a 4-character password on a tag to protect it from unauthorized writing.
I was wondering if they could be protected from reading as well. I've only skimmed through the N216 datasheets, but IIRC these have some encryption support built-in to do things like that.
I guess what I'm looking for is something like the Desfire EV1 public transport card we have here, AFAIK the data is cyphered and these cards cannot be copied.
I already did. Ordered a new ring with a discount, but I guess it will take a while to arrive. So I glued this one to use it until then. Afterwards I'll just keep it as a backup.
No, the 4-hour timeout is a different behaviour (and intended, it seems). That is, the Smart Lock feature disabling itself after 4 hours of inactivity. I think it takes longer than that.
On the other hand, the bug seems to cause the NFC radio to stop polling for tags after locking. So even though Smart Lock is enabled, it can't read them.
I've had the bug multiple times in less than an hour so it is definitely not the same as the stated timeout.
In any case, the Xposed module does nothing about the timeout. I still have to draw the pattern once every morning. But not during the day after that. Sometimes, it takes a bit longer to read the ring and the screen turns off before unlocking, but then it usually reads right away after turning on again, so it works for me. YMMV.