They only publish true stuff, yes, but that's not normally what one means by 'selective'. They're not a hacking organization, they don't go around hacking people to obtain stuff. And yes, someone gave them some Russian stuff not too long ago.

There's no denying they promote a certain narrative in their PR, but are there known examples of leaks withheld by Wiki Leaks, where the whistleblower had to go elsewhere?

Here is one example, where they refused to publish leaks from/about Russia:

http://foreignpolicy.com/2017/08/17/wikileaks-turned-down-le...

I recall another case when Wikileaks itself advertised a Russia leak and then never released anything.