SetAnimInstanceClass is broken in unreal 4.22. This is a project-level fix for it till 4.22.3 (or 4.23) comes out.

Unreal-AnimBP-4.22-hotfix.cpp

// Context: https://answers.unrealengine.com/questions/887556/view.html
//          This post references a key commit here: https://github.com/EpicGames/UnrealEngine/commit/c4ea32533ee98508ab68038488a09b3a12d54cd4#diff-73715a20583ca5ead5c94c9183679d47
//          This change forces evaluation to complete, before updating the animation class.
//          This happens during ClearAnimScriptInstance - BUT - the code path to this change, is not special.
//          That means it should be valid to do this BEFORE calling clear, i.e before even calling SetAnimInstanceClass itself!
//          So that's what we do. We simply wrap SetAnimInstanceClass in a function we can call, 
//          and inside that function we ensure evaluation is complete. This fixes it for me in all my use cases!

// Requires:
// - C++ in your project. 
//   I used a static global blueprint function library. 
//   You can put this function *anywhere* that works for you.

// 

//This is the cpp file implementation:

void UTypesAndGlobals::SetAnimInstanceClassFix(USkeletalMeshComponent* SkeletalMesh, UClass* NewClass) {
  // For details: https://gist.github.com/underscorediscovery/46ecaf5cc1ba97f1a7e922f6a77a5705
  // We may be doing parallel evaluation on the current anim instance
  // Calling this here with true will block this init till that thread completes
  // and it is safe to continue
  const bool bBlockOnTask = true; // wait on evaluation task so it is safe to swap the buffers
  const bool bPerformPostAnimEvaluation = true; // Do PostEvaluation so we make sure to swap the buffers back. 
  SkeletalMesh->HandleExistingParallelEvaluationTask(bBlockOnTask, bPerformPostAnimEvaluation);
  SkeletalMesh->SetAnimInstanceClass(NewClass);
}

//Header configuration for clarity and descriptive purposes:

UCLASS()
class MYPROJECT_API UTypesAndGlobals : public UBlueprintFunctionLibrary {
  GENERATED_BODY()
  public:
    
    UFUNCTION(BlueprintCallable, Category = "MyProject|4.22 AnimBP fix")
    static void SetAnimInstanceClassFix(USkeletalMeshComponent* SkeletalMesh, UClass* NewClass);

}