Is my bark mulch sour?

It's mulching time, and I've just slogged a heavy bag of small bark chips home, to find it smells rather less like woodland and more like a working farm.

I've read some conflicting view on the web about whether it is OK for bagged bark mulch to have a manure-like pong to it. Some sites say it is normal and goes after a while, whereas others suggest it is has gone sour or hasn't been stored correctly?

I think most garden centres store this stuff on pallets outdoors, can imagine they sit there soaking wet and are fermenting, so no doubt it is common. BUT I'm not sure that makes it acceptable!?