Backup regions were a bit of a misfeature. They’re exceptionally difficult to build a nice UX for, and what you ran into is one of the reasons why.
That said, when you run three regions you’re usually better off disabling backup regions entirely : fly regions backup ewr fra syd), then when you set a count set a max-per-region flag: fly scale count 6 --max-per-region 2.
It is very unlikely that we will lose a whole region, and if we do we’ll temporarily just route you somewhere a little slower in this setup. syd → hkg is not much of an improvement over syd → ewr.
We’re working on our scheduler options now. In the future, we should be able to let you define fallback rules with more precision.