Barbara… It’s near two years since you asked your question, and it seems nobody has answered. Your maintenance seems perfect. The Rosewood cracks I’ve experienced (in guitars) have all been along the grain. Yours appears to be on the dome of the beak with no relationship to the grain. Rosewood can be tricky stuff; but if it took three years to crack, and not along the grain, I don’t think the wood or manufacture caused it. 50% humidity is perfect by the way. My guess would be an ‘event’ like an incredibly saturated block or and exposure to greater than normal heat or even cold, where the block’s wood (often cedar) expanded or contracted at a greatly different rate than the Rosewood. But I’d imagine there’s no way to truly tell.
You didn’t mention any change in the playing like new difficulties with the extreme high or low notes. If that’s the case I wouldn’t worry much. Your choice would be to fix or not fix the crack. I’m a believer in not repairing a crack that doesn’t impair any notes and that isn’t continuing to spread. Given that this post is so old, you might compare the crack to the photo you posted back then. If the crack’s the same, I’d say leave it alone. But I’m in the minority in that opinion, though I suspect many repair people of not being able to turn away a paying job.
If done – and if the crack closes when clamped, the repair would be to let the instrument dry for about a week and micro-inject (insulin needles?) Crazy Glue and clamp the crack closed. Some have the skill to use the thinnest Crazy Glue on an already clamped crack and let the glue migrate in.
If the crack is resistant to closing when clamped – and this is a repair I have seen create new problems with time – some would fill it either with a color-matched wood putty or some sawdust made from a non-functional and not visible part of the instrument. Really good repair persons often keep small jars of different colored sawdust from earlier projects. There is though the future potential in this type of repair for the crack to naturally close. But the crack’s now plugged. The original crack might then need to get longer or deeper going around the repair that now acts as a wedge in the opening. I’ve seen this happen, especially in apartment houses, which can quickly go to extreme dryness in Winter.
Too much info? Likely. But it’s everything within my experience. If after all these years the crack looks the same and the instrument plays well… I’d say leave it alone.
Your posting was like a note in a bottle thrown into the sea. For what it’s worth, an answer! — k