I know that overlifting is possible from personal experience. I should have stopped at ONE hair but I kept going. I have done it correctly on two others and it's a teeny tiny lift. You're on the right track - you found some gunk! But are we talking manual or auto?
Rotating two shorties for six weeks means each one has been
through what...40 or 80 or more charge cycles? (Guessing here.) I think they are supposed to last 200 cycles, but not positive about it.
Your kit is still new and if it was me, I'd definitely open a CS ticket.