When the astrologist is looking for a material traded in the island markets I've noticed that there's an increase in all trades involving that material instead of only trades that offer the material. I would think the astrologist should actually decrease trades requesting the looked for material.

(If this issue isn't a bug or exists only in my imagination, could this thread please be moved to suggestions?)

Oh no, you're right about it existing. I've witnessed this before when using the Astrologist to look for Vermillion. This isn't in your imagination! Whether or not it's a bug though I don't know.

I'd kind of suspected this for a while, but previously either I couldn't do all that much exploring or I was looking for premium items. It's only now that I've got a significant watchflock and started looking for vermillion to sell that I became sure it wasn't my imagination.

In the case of the highest level astrologist set to vermillion it seems like the trades are about equal thirds of no vermillion involved, vermillion requested and vermillion offered.

Astrologist indeed increased all types of trades. Thank you for reporting, this problem should be gone.