Thanks. That looks also correct, so we have to dig deeper.
Can you run this debug macro with root -b -q debug_macro.C and paste the output here? Something seems to be going wrong with the ROOT plugin system.
void debug_macro() {
gDebug = 3;
auto h = gROOT->GetPluginManager()->FindHandler("ROOT::Math::Minimizer", "Minuit2");
h->LoadPlugin();
auto * min = (ROOT::Math::Minimizer *)( h->ExecPlugin(1,"migrad" ) );
std::cout << min << std::endl;
}