Could just be RNGesus cursing you.
I was out mining with just 64 tons cargo rack yesterday. Filled it with opals in just about an hour for 105 mil. It was my first time in an opal hotspot but it seems hotspots only have a slight increase in opal spawning chances. Out of 5 asteroids i had 3 opals, 1 grandidierite and 2 bromellite.
I noticed in rings without hotspots there's often like localised hotspots. If you find an opal asteroid (or any other ore) the closest asteroids are often the same ore too, so it's not rare to mine 2 or 3 asteroids in a row with the same minerals if you dont wander too far from the first.