This bug was introduced in 4.11 but is present in all version after as well, I also tested this on 4.10.4 and this bug isn’t present.
Its going to be a hard bug to filter out but it will be easier showing a video of the bug.
There are no errors when this happens but i can produce a engine crash when using a loot filter in my project when this UMG bug is in affect it may help or not?
Assertion failed: IsValid() [File:D:\Build\++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\Core\Public\Templates\SharedPointer.h] [Line: 739]
UE4Editor_Core!FDebug::AssertFailed() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:440]
UE4Editor_UMG!UComboBoxString::HandleSelectionChanged() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\umg\private\components\comboboxstring.cpp:218]
UE4Editor_UMG!TBaseUObjectMethodDelegateInstance<0,UComboBoxString,TTypeWrapper<void> __cdecl(TSharedPtr<FString,0>,enum ESelectInfo::Type)>::Execute() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:760]
UE4Editor_UMG!TBaseUObjectMethodDelegateInstance<0,UComboBoxString,void __cdecl(TSharedPtr<FString,0>,enum ESelectInfo::Type)>::ExecuteIfSafe() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:850]
UE4Editor_UMG!TBaseDelegate<void,TSharedPtr<FString,0>,enum ESelectInfo::Type>::ExecuteIfBound() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegatesignatureimpl_variadics.inl:608]
UE4Editor_UMG!SComboBox<TSharedPtr<FString,0> >::OnSelectionChanged_Internal() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\public\widgets\input\scombobox.h:325]
UE4Editor_UMG!TBaseSPMethodDelegateInstance<0,SComboBox<TSharedPtr<FString,0> >,0,TTypeWrapper<void> __cdecl(TSharedPtr<FString,0>,enum ESelectInfo::Type)>::Execute() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:321]
UE4Editor_UMG!TBaseSPMethodDelegateInstance<0,SComboBox<TSharedPtr<FString,0> >,0,void __cdecl(TSharedPtr<FString,0>,enum ESelectInfo::Type)>::ExecuteIfSafe() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegateinstancesimpl_variadics.inl:428]
UE4Editor_UMG!TBaseDelegate<void,TSharedPtr<FString,0>,enum ESelectInfo::Type>::ExecuteIfBound() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\public\delegates\delegatesignatureimpl_variadics.inl:608]
UE4Editor_UMG!SListView<TSharedPtr<FString,0> >::Private_SignalSelectionChanged() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\public\widgets\views\slistview.h:618]
UE4Editor_UMG!STableRow<TSharedPtr<FString,0> >::OnMouseButtonUp() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\public\widgets\views\stablerow.h:449]
UE4Editor_Slate!<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1>::operator()() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4598]
UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_8d0e9a1da76abd0a756a3a9d775f5ed1> >() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:215]
UE4Editor_Slate!FSlateApplication::RoutePointerUpEvent() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:4587]
UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5041]
UE4Editor_Slate!FSlateApplication::OnMouseUp() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\slate\private\framework\application\slateapplication.cpp:5021]
UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:1510]
UE4Editor_Core!FWindowsApplication::DeferMessage() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:1850]
UE4Editor_Core!FWindowsApplication::ProcessMessage() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:745]
UE4Editor_Core!FWindowsApplication::AppWndProc() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsapplication.cpp:667]
user32
user32
UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\windows\windowsplatformmisc.cpp:903]
UE4Editor!FEngineLoop::Tick() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\launchengineloop.cpp:2729]
UE4Editor!GuardedMain() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\launch.cpp:148]
UE4Editor!GuardedMainWrapper() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
UE4Editor!WinMain() [d:\build\++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:200]
UE4Editor!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:264]
kernel32
ntdll