Launch on an iOS device from withing the debugger
Watch the log for output such as the callstack attached
Main Thread Checker: UI API called on a background thread: -[UIView bounds]
{{ PID: 14347, TID: 9796350, Thread name: (none), Queue name: com.apple.root.default-qos.overcommit, QoS: 0}}
{{ Backtrace:}}
{{ 4 Project 0x00000001063daa9c FDisplayMetrics::RebuildDisplayMetrics(FDisplayMetrics&) 232}}
{{ 5 TheProject 0x00000001084d5d4c UGameEngine::DetermineGameWindowResolution(int&, int&, EWindowMode::Type&, bool) 428}}
{{ 6 TheProject 0x00000001084d5a74 UGameEngine::ConditionallyOverrideSettings(int&, int&, EWindowMode::Type&) 100}}
{{ 7 TheProject 0x0000000108524c64 UGameUserSettings::PreloadResolutionSettings() 748}}
{{ 8 TheProject 0x000000010535be48 FEngineLoop::PreInitPreStartupScreen(char16_t const*) 9980}}
{{ 9 TheProject 0x000000010534b520 FAppEntry::Init() 88}}
{{ 10 TheProject 0x00000001063ddff4 -[IOSAppDelegate MainAppThread:] 288}}
{{ 11 Foundation 0x00000001b685e9d0 1A46239D-F2FC-34B6-95BC-9F38869F0C85 1272272}}
{{ 12 libsystem_pthread.dylib 0x00000001b618ad98 _pthread_start 156}}
{{ 13 libsystem_pthread.dylib 0x00000001b618e74c thread_start 8}} 2020-07-13 11:56:31.718594 1000 TheProject[14347:9796350] [reports] Main Thread Checker: UI API called on a background thread: -[UIView bounds]
{{ PID: 14347, TID: 9796350, Thread name: (none), Queue name: com.apple.root.default-qos.overcommit, QoS: 0}}
{{ Backtrace:}}
{{ 4 TheProject 0x00000001063daa9c FDisplayMetrics::RebuildDisplayMetrics(FDisplayMetrics&) 232}}
{{ 5 TheProject 0x00000001084d5d4c UGameEngine::DetermineGameWindowResolution(int&, int&, EWindowMode::Type&, bool) 428}}
{{ 6 TheProject 0x00000001084d5a74 UGameEngine::ConditionallyOverrideSettings(int&, int&, EWindowMode::Type&) 100}}
{{ 7 TheProject 0x0000000108524c64 UGameUserSettings::PreloadResolutionSettings() 748}}
{{ 8 TheProject 0x000000010535be48 FEngineLoop::PreInitPreStartupScreen(char16_t const*) 9980}}
{{ 9 TheProject 0x000000010534b520 FAppEntry::Init() 88}}
{{ 10 TheProject 0x00000001063ddff4 -[IOSAppDelegate MainAppThread:] 288}}
{{ 11 Foundation 0x00000001b685e9d0 1A46239D-F2FC-34B6-95BC-9F38869F0C85 1272272}}
{{ 12 libsystem_pthread.dylib 0x00000001b618ad98 _pthread_start 156}}
{{ 13 libsystem_pthread.dylib 0x00000001b618e74c thread_start 8}}
{{ Main Thread Checker: UI API called on a background thread: -[UIView contentScaleFactor]}}
{{ PID: 14347, TID: 9796350, Thread name: (none), Queue name: com.apple.root.default-qos.overcommit, QoS: 0}}
{{ Backtrace:}}
{{ 4 TheProject 0x00000001063daa30 FDisplayMetrics::RebuildDisplayMetrics(FDisplayMetrics&) 124}}
{{ 5 TheProject 0x000000010534b2dc FAppEntry::PlatformInit() 216}}
{{ 6 TheProject 0x0000000105e58c58 FIOSPlatformMisc::PlatformInit() 52}}
{{ 7 TheProject 0x000000010535bfc0 FEngineLoop::PreInitPreStartupScreen(char16_t const*) 10356}}
{{ 8 TheProject 0x000000010534b520 FAppEntry::Init() 88}}
{{ 9 TheProject 0x00000001063ddff4 -[IOSAppDelegate MainAppThread:] 288}}
{{ 10 Foundation 0x00000001b685e9d0 1A46239D-F2FC-34B6-95BC-9F38869F0C85 1272272}}
{{ 11 libsystem_pthread.dylib 0x00000001b618ad98 _pthread_start 156}}
{{ 12 libsystem_pthread.dylib 0x00000001b618e74c thread_start 8}}2020-07-13 11:56:43.014776 1000 TheProject[14347:9796350] [reports] Main Thread Checker: UI API called on a background thread: -[UIView contentScaleFactor]
{{ PID: 14347, TID: 9796350, Thread name: (none), Queue name: com.apple.root.default-qos.overcommit, QoS: 0}}
{{ Backtrace:}}
{{ 4 TheProject 0x00000001063daa30 FDisplayMetrics::RebuildDisplayMetrics(FDisplayMetrics&) 124}}
{{ 5 TheProject 0x000000010534b2dc FAppEntry::PlatformInit() 216}}
{{ 6 TheProject 0x0000000105e58c58 FIOSPlatformMisc::PlatformInit() 52}}
{{ 7 TheProject 0x000000010535bfc0 FEngineLoop::PreInitPreStartupScreen(char16_t const*) 10356}}
{{ 8 TheProject 0x000000010534b520 FAppEntry::Init() 88}}
{{ 9 TheProject 0x00000001063ddff4 -[IOSAppDelegate MainAppThread:] 288}}
{{ 10 Foundation 0x00000001b685e9d0 1A46239D-F2FC-34B6-95BC-9F38869F0C85 1272272}}
{{ 11 libsystem_pthread.dylib 0x00000001b618ad98 _pthread_start 156}}
{{ 12 libsystem_pthread.dylib 0x00000001b618e74c thread_start 8}}
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-95916 in the post.