Snap build service picking default branch instead of pushed branch

I have my GitHub repo hooked up to the snapcraft build service such that when I push any new commits, it triggers a new build. This is just using the default way of connecting repos to the build service via the web hook. However, it seems as though when I push to a branch, the triggered build uses the default branch instead of the pushed one. Is this a bug in how the web hook is defined (it was established by clicking on the “Set up webhook” link on the Settings tab of the page for the snap), or is there some extra setting I need to tweak somewhere to get this to work as expected?

Build log for one example can be found here: