On a related note. The only Julia-native global solver that I am aware of is Alpine.jl. It is not yet working with JuMP v0.19+, but you can track that progress here, https://github.com/lanl-ansi/Alpine.jl/pull/149.
In the case of AC-OPF, last I recall, Alpine did not support trig functions yet, so it would not help you in that case.
If you switch to the rectangular voltage form of the AC-OPF you could then use Alpine or Gurobi v9 to get global optimality proofs for small to medium sized problems.