If you directly connect a state which has another state machine to the entry of State Machine of Animation BP, a crash will occur when changing parameters.
If you connect an empty State to an Entry and pass through that State, no crash will occur.
UE4Editor_Engine!FAnimNode_StateMachine::GetStateInfo() [animnode_statemachine.cpp:312] UE4Editor_Engine!FAnimInstanceProxy::GetRelevantAssetPlayerFromState() [animinstanceproxy.cpp:1509] UE4Editor_Engine!UAnimInstance::GetRelevantAnimTimeFraction() [animinstance.cpp:2601] UE4Editor_Engine!UAnimInstance::execGetRelevantAnimTimeFraction() [animinstance.h:346] UE4Editor_CoreUObject!UFunction::Invoke() [class.cpp:4515] UE4Editor_CoreUObject!UObject::CallFunction() [scriptcore.cpp:732] UE4Editor_CoreUObject!UObject::execLet() [scriptcore.cpp:1953] UE4Editor_CoreUObject!UObject::ProcessInternal() [scriptcore.cpp:954] UE4Editor_CoreUObject!UObject::CallFunction() [scriptcore.cpp:856] UE4Editor_CoreUObject!UObject::ProcessInternal() [scriptcore.cpp:954] UE4Editor_CoreUObject!UFunction::Invoke() [class.cpp:4515] UE4Editor_CoreUObject!UObject::ProcessEvent() [scriptcore.cpp:1314] UE4Editor_Engine!FExposedValueHandler::Execute() [animnodebase.cpp:580] UE4Editor_Engine!FAnimNode_StateMachine::FindValidTransition() [animnode_statemachine.cpp:611] UE4Editor_Engine!FAnimNode_StateMachine::Update_AnyThread() [animnode_statemachine.cpp:395] UE4Editor_Engine!FPoseLinkBase::Update() [animnodebase.cpp:233] UE4Editor_Engine!FPoseLinkBase::Update() [animnodebase.cpp:233] UE4Editor_Engine!FAnimNode_StateMachine::Update_AnyThread() [animnode_statemachine.cpp:516] UE4Editor_Engine!FPoseLinkBase::Update() [animnodebase.cpp:233] UE4Editor_Engine!FAnimInstanceProxy::UpdateAnimationNode() [animinstanceproxy.cpp:41] UE4Editor_Engine!FAnimInstanceProxy::UpdateAnimation() [animinstanceproxy.cpp:904] UE4Editor_Engine!USkeletalMeshComponent::PerformAnimationEvaluation() [skeletalmeshcomponent.cpp:1655] UE4Editor_Engine!FParallelAnimationEvaluationTask::DoTask() [skeletalmeshcomponent.cpp:103] UE4Editor_Engine!TGraphTask<FParallelAnimationEvaluationTask>::ExecuteTask() [taskgraphinterfaces.h:784] UE4Editor_Core!FTaskThreadAnyThread::ProcessTasks() [taskgraph.cpp:910] UE4Editor_Core!FTaskThreadAnyThread::ProcessTasksUntilQuit() [taskgraph.cpp:786] UE4Editor_Core!FTaskThreadBase::Run() [taskgraph.cpp:502] UE4Editor_Core!FRunnableThreadWin::Run() [windowsrunnablethread.cpp:76]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-53444 in the post.