PostbackUrl messing up UpdatePanel async postback.

Question

So here is the situation.

I have a page with two custom controls. One of these controls contains an UpdatePanel, and this has worked correctly for a very long time.

There is another user control which has two LinkButtons, that have recently been switched over to use PostbackUrl instead of the simple . Having a LinkButton with a PostbackUrl is causing the UpdatePanel to receive a full postback.

This is very odd, I'm not sure what's causing the issue. How does PostbackUrl on a control affect the postback of the entire page?

Any ideas?

EDIT

I don't know if this will make any difference, but the LinkButton's are nested within a LoginView control, and the PostbackUrl property is set in the codebehind.