I noticed this in my server with my friends (It's hosted by a third-party source, so it probably isn't something with my computer) and started testing in my own creative world. As you can see, this Warden was trapped inside this box made out of wool, so it wouldn't be provoked by anything, and didn't burrow for well over 60 seconds. Is this how it's meant to be? Every search-result I get says it should go away after about 60 seconds, but it just won't! 

Finally, after 6 full minutes, it burrowed away, but I don't think that's how it's meant to work...