FLV seeks slowly after cutting with FFMPEG

The input video is a h264 flv and is able to seek instantly to any keyframe.

When I cut a snippet with the following command, keeping the flv container, the resulting video takes a few seconds to seek to any keyframe.

ffmpeg -y -ss 00:27:31 -i input.flv -to 00:58:27.5 -c copy -copyts output.flv

When I cut a snippet with the following command, using an mp4 container, the resulting video retains its ability to seek instantly to any keyframe.

ffmpeg -y -ss 00:27:31 -i input.flv -to 00:58:27.5 -c copy -copyts output.mp4

Why does the output.flv video lose its abilty to seek instantly?

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