skipping with cursor keys in files that still grow wrongly detects "end of file"

[muellaa@…]

in one terminal:
$ mplayer -dumpstream ​http://some.stream.server.org/radio.mp3

wait some seconds, then in different terminal:
$ mplayer stream.dump

This produces a time shift playback.
It works nicely including skipping forward and backward with cursor keys.
However, only up to the position that is defined by the file size valid while issuing the command.
mplayer quits with "end of file" if the target position of skipping lies behind the "original" file-length even though the file has increased sufficiently.