I expect what’s happening is that 8 car trains are stopping at the correct stop car mark, which happens to be colocated with the 10 car board.
All trains starting from London terminals, with passenger access from the buffer stops end of the platform, suffer from the problem of more people squeezing into the rear most carriages.
That is what is happening, but only because the 8-car mark has been removed.
Yes, a busy rear carriage is to be expected, but it is made worse because platforms are announced at Waterloo so late, meaning that there is not time for people to distribute themselves along the platform before the train arrives, and also because the exits at two of the busiest stations - Raynes Park and Kingston - are now adjacent to the eighth carriage of the train instead of the antepenultimate (i.e the sixth of an eight car train), so regulars tend to go for that carriage in order to be ahead of the scrum at the barrier (especially at Kingston where there are far fewer gates than needed). Also at Kingtson it puts the rear carriage next to the narrowest part of the platform, making it difficult for anyone trying to join the train to fight their way past the people leaving it. And it means that less of the train is under the platform canopy.
There is a related issue at Wimbledon, where the platform exits are at the country end of the platforms - that is, the rear for up trains. Staff can shout all they like for intending passengers to move down the platform, but regulars know the only chance of getting on a peak hour service is to wait near the entrance, because most people alighting at Wimbledon (a lot do, to change for the Tube, trams or Thameslink) will be in the rear carriage in order to be first up the stairs - queuing to get off the platform, and possibly missing your connection, being the norm there because the stairs are so narrow. A second bridge is badly needed there.
Isn't the solution to move the 8 car board rearwards so that people spread themselves further forwards? Or does that cause other issues?
There isn't an 8-car board. There used to be, but it was removed a few years ago. No-one knows why.
I was given an excuse that drivers might forget how long their train was and stop in the wrong place, but that doesn't explain why there is still a 4/5 car board. And surely, a driver is hardly likely to forget whether he is driving a 455 or 450, rather than a 458, 701 or 707. (I suppose when 456s were still around, there was a possibility of a driver in a 455 cab forgetting whether he had a 456 on the back, but that is why guards have been kept on SWR - so they can check the train is fully on the platform before releasing the doors)