I am using UNC mode so 'BufferingDelayInMilliSeconds' registry may not have any effect with it.
It happened again after like 15 mins even still have timeshift left, so I move back 1 min again and fine again. I tried move forward 15 sec and 15 sec still OK.
Latest 1.20It does have an effect (irrespective of source - local file/UNC path/RTSP).
Every time you skip around the buffer inside TsReader is rebuilt i.e. playback is effectively re-started from the new place in the stream.
Which version of MP are you using?