Right so noticed this when file was set to MKV but it also happens with Pass (.ts)
Lets say you record a program on between 20.00 - 21.00, but start to watch it at 20.30, if you just watch it, its fine, but if you fast fwd upto 20.30 it seems to work, but fast fwd , skip after this point it will go up a minute or 2 before then rewinding, you can use the jump fwd 10 mins to get past it, but then rewind or fastfwd it soon jumps back to 20.30.
I did find a couple of instances on google where it was reported to do this, but these where along time ago