See linked UDN for more info.
When a server initiates a non-seamless travel, it will inform any connected clients by sending the reliable ClientTravelInternal RPC. After a delay (controlled by ServerTravelPause), it will then shutdown its NetDriver before loading the next map. However, as part of UNetDriver::Shutdown, it will also send a NMT_Failure control channel message to all clients.
Normally, the clients will have already started travelling and shutdown their own NetDrivers when this message would arrive. With unideal network conditions though, it may be possible for the client to receive this network failure before starting its travel, resulting in the client permanently disconnecting itself from the server.
In a multiplayer game (with the GameMode's bUseSeamlessTravel set to false), initiate a server travel. To more reliably reproduce the issue, set the NetDriver's ServerTravelPause to 0, and use bad network emulation settings.
Expected: The clients and server disconnect, the server travels to the new map, and the clients then reconnect.
Actual: The clients can sometimes receive a network failure message, causing them to permanently disconnect from the server.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-160505 in the post.