Spawning a warden normally (with /summon warden or just a spawn egg) will cause them act normally, but as soon as you try summoning one with any NBT data, including simply:

/summon warden ~ ~ ~ {}

the warden will immediately perform its dig action and despawn.

I seem to have figured out why this is happening: if you run the following command - 

/data get entity @e[type=warden,limit=1,sort=nearest] Brain.memories

on a normal warden, it will display one memory titled "dig_cooldown". However, running this same command on a warden summoned with NBT (you've gotta be fast, it will be actively digging away) will display a memory that's instead titled "sniff_cooldown". This memory appears to control when the warden can sniff, and some code relating to it may have been changed when the Sniffer was added (because the sniffer has a memory of exactly the same name that does something different).