If you don’t wanna modify engine code, luckly it is accesible via reflection so just slap this somewhere i just put it on StartupModule.
#if WITH_EDITORONLY_DATA
auto* rootMotionStruct = FRepRootMotionMontage::StaticStruct();
auto* brokenProperty = rootMotionStruct->FindPropertyByName(TEXT("AnimMontage"));
brokenProperty->SetPropertyFlags(brokenProperty->PropertyFlags | EPropertyFlags::CPF_RepSkip);
#endif