Yup, Single Pristine HotSpots. The theory was that double or triple made no difference to Core Mining. The number of Cores remained the same but the percentage of Cores for that mineral was greater in a Hot Spot. Now that bias seems to have been drastically reduced if not eliminated all together.Was this a single LTD hotspot? I've found similar. Went prospecting for an hour or so in Alectrona (one of the gas giants has a single LTD hotspot) shortly after I bought my FC. I think I found a single LTD core.
It does seem that FD seem to have changed something that has had a detrimental effect - I used to do quite well mining in single hotspots.
Having pumped SubSurface Mining it's as if FDev have decided that all other forms of mining should be nerfed.