Packages that have been accepted by the archive admins for the -proposed queues which are a testing ground for Stable Release Updates (SRUs) come in with a lower build priority than packages in the main development branch. During the beginning of a development cycle, this causes proposed packages to get blocked behind the large numbers of packages being built, such that it requires manual archive admin intervention to raise their priority so they can be built, installed, and tested by users before being released to -updates. It's particularly frustrating for SRU testers to go to the effort of reproducing a bug and then realize that the proposed package has been accepted but is not available in the archive because it has been waiting to build for 18 hours.
Given that the small number of packages coming through proposed relative to those in the main devel release, defaulting the build priority of proposed packages to be higher than that of the devel release would not significantly delay development packages.
Right, sources to -proposed and -backports pockets get lower priority than the ones in the release pocket.
|| Pocket || Score parcel ||
|| -proposed || 0 ||
|| -backports || 1500 ||
|| <release> || 3000 ||
|| -updates || 4500 ||
|| -security || 6000 ||
So the behavior you reported is correct. Let's get some feedback from the distro-team about your proposal, if they agree, we can easily implement it.