You're the scheduler expert and know the code better than anyone, but if this actually works I think we need to rework the documentation a little bit.
Section 18.104.22.168 of the 14.2.1 Admin Guide says:
You can create a formula that the scheduler uses to sort jobs. The scheduler applies this formula to all jobs in the complex, using it to calculate a priority for each job. For example, you can specify in the formula that jobs request- ing more CPUs have higher priority. If the formula is defined, it overrides fairshare and sorting jobs on keys.
To me, "overrides" implies that those other options are ignored, not that "the formula is the primary key". I could see where you might interpret it otherwise, but we should probably be absolutely clear.
Similarly 4.8.21 says:
This formula will override both job_sort_key and fair_share for sorting jobs. If the job_sort_formula server attribute contains a formula, the scheduler will use it. If not, and fairshare is enabled, the scheduler computes job priorities according to fairshare. If neither the formula nor fairshare is defined, the scheduler uses job_sort_key.
This would seem (to me at least) to imply that both fairshare and the sort keys are ignored if the formula is specified.