If a level contains a landscape actor, and if you stream multiple instances of that level into your map, you will get errors in the message log that the landscape actors have the same GUID.
Found in 4.20 CL# 4369336, 4.21 CL# 4753647, 4.22 CL# 5660361, 4.23 CL# 5898046
Results:
Map warnings will appear about having the same GUID
Expected:
No errors would appear
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-72811 in the post.