Hey! Listen, I'm going to be direct with you: what you're describing is a red flag for gray mold (the infamous Botrytis).
Even if you don't see any white fuzz or "moss" yet (the final stage of the spores), your intuition is spot on. Here's my analysis of the situation for your buds:
1. Why "wet brown" is a bad sign
The fact that the leaves aren't dry/burnt, but soft and brown, is the typical symptom of Botrytis attacking from the inside out.
The ultimate test: Take one of the small brown leaves and gently pull on it. If it comes off easily, with no resistance and a slightly sticky base, it's 100% bud rot.
The trap: When the white fuzz appears, it's already too late; the spores are spreading all over your grow box.
2. The "partial harvest" factor
It's a classic. By harvesting the top of the plant, you've created "wounds" on the stem and altered the airflow.
The lower buds, which are already denser and less ventilated, end up with stagnant humidity.
It's the perfect spot for mold to settle in, especially if the humidity spiked after the first cut.
3. What I would do in your place (Immediate action)
Don't wait until tomorrow, act now to save the rest:
Precision surgery: Cut the affected bud (and even a bit lower on the stem) with disinfected scissors.
Direct trash bag: Don't shake the branch in the box! Put a bag around the bud before cutting to trap the spores.
Complete check:
Inspect all the other lower buds. Gently move the flowers aside to look near the main stem. If it's brown or gray inside: we cut.
Boost the ventilation: Your extractor should be running at full blast and your fans should be circulating air more than ever. You need to bring the humidity down below 45-50%.
My verdict: This is likely the start of a mold issue. Don't take any risks smoking that; it's bad for your lungs. Sacrifice that branch to save the rest of your harvest!
I hope I could help you. Have a good day and stay strong!