Placing a FunctionalTest Blueprint into a level that contains a landscape component and a sub-level that also contains a landscape component results in a crash when PIE is started. This only occurs in 4.20+ if the project is being upgraded from 4.19 or earlier. The crash does not occur if the project is created in 4.20, or if the FunctionalTest Blueprint is not present in the level. The crash also does not occur when upgrading a project from 4.18 to 4.19.
This appears to be due to the LandscapeComponent not having a MobileCombinationMaterialInstance set. The Editor attempts to create this, which ends up resulting in an assert.
The Editor crashes with the error Cannot call UnrealScript (BP_SampleTest_C /Game/VehicleAdvCPP/Maps/UEDPIE_0_VehicleAdvExampleMap.VehicleAdvExampleMap:PersistentLevel.BP_SampleTest_2 - Function /Script/FunctionalTesting.FunctionalTest:DebugGatherRelevantActors) while PostLoading objects
The Editor starts PIE successfully.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-65669 in the post.