It’s likely your new sheep are still in range of two of your other sheep, so both are capped at 8, they’re just sharing two sheep to reach that number. I’m not sure what the precise range is or how the game calculates it though. I tried to measure it once but didn’t manage to find a clear number, 8 squares distance is still very close though in my experience, I’m surprised you were even able to breed 6 that close.
You can try using deep water to set up temporary pens and nudge the sheep over until you find an appropriate distance and then build a proper farm, or just build and break as you go if mats aren’t a concern.