Warden doesn't dig back to ground

I summoned a Warden (from a shrieker) and went away doing other things. When I came back, he was still there.

 

Steps to reproduce:

1. Spawn a Warden
2. Wait

Expected behaviour:

Warden should dig back into ground, when not angered by anything for 60 seconds. (I checked that, he haven't heard anything, his ears didn't lighted up.