Apologies if this is in the wrong location.
The newest Package Resolver Tool 0.1.27 does not work on Unity 2019 due to a change in VRC.PackageManagement.Resolver.Resolver.ForceRefresh. 0.1.27 is being offered to Unity 2019 projects even if "Show Pre-Release Packages" is disabled. The previous version 0.1.26 works just fine.
Perhaps 0.1.27 should be marked incompatible with 2019 in some way, or the change in ForceRefresh should be gated behind a unity version define check.