It's normal. Uncompressed video doesn't have temporal dependencies between frames, so you can seek instantaneously wherever you want in an uncompressed video. The same would be true if you were encoding with mjpeg (all frames are intra), or ASP/AVC with only intra frames (max-keyint set to 1 with x264 for example).
As for when to use open gop : always. There are no (valid) reason not to use it. It's better, period.
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