Constraints violated when allocation failed + rescheduled?

Nomad version

Nomad v1.1.0 (2678c3604bc9530014208bc167415e167fd440fc)

Operating system and Environment details

Ubuntu 18.04

Issue

We have a job with these constraints/affinities:

{"Constraints"=>
  [{"LTarget"=>"${meta.fly_region}",
    "Operand"=>"distinct_property",
    "RTarget"=>"1"},
   {"LTarget"=>"${meta.fly_region}",
    "Operand"=>"set_contains_any",
    "RTarget"=>"atl,cdg,gru,hkg,iad,lhr,maa,nrt,sjc,syd"}],
 "Spreads"=>
  [{"Weight"=>100, "Attribute"=>"${meta.fly_region}", "SpreadTarget"=>nil}],
 "Affinities"=>
  [{"Weight"=>50,
    "LTarget"=>"${meta.fly_region}",
    "Operand"=>"set_contains_any",
    "RTarget"=>"atl,cdg,gru,hkg,iad,lhr,maa,nrt,sjc,syd"}]}

When the job is created or updated, it does what you'd expect (one allocation per fly_region). It's behaving weirdly when allocations fail, though, and violating the distinct_property constraint when it reschedules them. Two allocations failed over night and the replacements ended up creating a situation where 2x allocs were running on clients with fly_region == 'atl' and fly_region == 'maa'.

Reproduction steps

I can't reproduce this on purpose but it's happened at least twice with the same job spec.

Is this actually a bug or am I misunderstanding how this is supposed to work?

No source file, test, or entry point is named. Start by tracing Nomad's scheduler behavior for failed allocations and rescheduling, focusing on the distinct_property constraint and fly_region metadata. Reproduce the reported job configuration if possible and verify whether replacements preserve one allocation per region.