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In PIE games, it looks like this may be caused by the window immediately taking control of the mouse and setting up focus.
In networked games, clients call ULocalPlayer::SpawnPlayActor which will create a default APlayerController (unless otherwise configured). This later gets deleted, but may have previously set focus.
EXPECTED: Before and after the release mouse capture function is called, the camera does not change unless you grab the left virtual joystick.
ACTUAL: The camera changes and tracks the mouse before the release function is called. After the function is called, it works like expected.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-39681 in the post.