Two-finger swipe 'breaks' heads-up notifications on Lollipop?
This is a bug, and it has been reported on AOSP Issue Tracker #79583.

As of current writing, the issue carries on to 5.0.1 Lollipop, reproduced on a Nexus 5. One claimed that the issue has gone on 5.0.1 (but didn't mention the device), and the rest still reported the same issue on it.

If you are interested in the follow-up, you might "star" the issue (I have done it myself).