In kde 5, when I start a pinned launcher, the icon moves into the taskbar, and hides from the pinned-launchers, until the application quits. In kde 6, my taskbar starts to get full now very early, when I start some of my pinned applications. I did not find an option to revert the behaviour like before.
Sorry, I now discovered, that the option "keep launchers separated" does what I want. But it does not apply immediately, I had to restart some applications to see it working, so I first thought, it does something else. Perhaps this could be a reminder, to fix this (apply option immediately, or at least display a warning about that)
I'm afraid this was an intentional change in Plasma 6, sorry. Sounds like you've already found the option to go back to the old thing.