I keep getting these spots developing on my oldest mother, the MAC. She's probably rootbound, it's been about 4 months since she potted up. I need to keep her in a cup for space, will deficiency get better if I just trim about 50% roots and plant back in a cup with fresh soil?
I would try and invent in a bigger cup. Looks like it wants more phosphorous or some calcium. maybe give it a nice flush if you don't plan on transplanting up in size. Then some fresh feed. could be a buildup in the medium given how small the pot is. If you root prune use clean shears and sterile soil or you might shock it to a early death,
Good Luck!
So far so good, looks great! Wish you fun and good luck with all your bonsai grows, very charming and I look forward to see the next update. Happy growing!