New maple leaves yellow, tips brown/dry

For starters, pretty new to bonsai, so still learning the basics (watering, fertilizing, etc.). Here's the story:

I've had this maple for a couple years now, and it's been relatively healthy until this year. I applied some N-rich fertilizer at the start of the growing season, and the first flush of leaves came out slightly pale, but otherwise healthy. Same for another maple I have, though its leaves were darker.

Later, new leaves at growth tips had a weird fleshy shade without a hint of green in it. Some of these new leaves started turning brown at the tips, despite paying attention to water appropriately as it got hotter. I panicked, and deciding it was getting to be an appropriate time of year to defoliate, I did so (probably a rash decision, please don't crucify me lol). I had some peace of mind from the dreaded verticillium for a few weeks, as the tree is still alive and producing new buds.

But now, as those new buds sprout, they're doing the same thing! Fleshy-color leaves. Some are also browning at the tips, but otherwise the tree seems healthy to me. I need a maple doctor. As someone in the medical field, here are my DDx:

- Overwatering (I use a good bit of organic medium since I live in a hot subtropical climate, but I might have overdone it)
- Nutrient imbalance (I've read about chlorosis from nitrogen/iron problems)
- Infection (fingers crossed, not this) 

Now 2 questions:
1. Can anybody help me with my problem? I'm worried that, if the issue is care-related, I might end up killing my healthy maple too lol.
2. Is there a good "diagnostic guide" for maple issues based on leaf "symptoms" that I can use in the future?

Far too young a plant to defoliate, and Maples are never fully defoliated. Lack of water is the usual reason for browning edges of leaves. Now you need to give it some shade in the afternoon, but keep it watered. Next year same, but do not remove leaves you need it to grow. Defoliating stops growth. Fertilise from late bud break.

Thanks, that makes sense about the defoliation. I'll try the shade, watering, and fertilizing you recommended and give a progress update in a month or two.