Just started a druid and on more than one occasion I shifted back to human form during combat. I did not choose to do this. Isn't the shape shifting supposed to last the entire encounter?

No. It lists a duration at the ability (dont know your specific one as I am sure its duratio is, as all durations are, Intelligence-based). But in longer battles it CAN run out.

 

Attention: As the "Crash at transition" bug prevents me from playing beyond a certain point (worked once for Gilded. W. but then crashed again) I can only assume what I saw on my brothers PC is still valid (2 days ago).

 

if not: Sorry.

I can't find the druid spiritshape duration anywhere. It does not list it at all on the spell detail page.

You are correct, I must have imagined it....oO

 

And on a Druid Test myself I ahve to say: it runs out to quickly, as some other people also wrote.