Either a bug or you simply are not actually in the hotspot. I can't recollect the system name where I mine platinum, but it's a popular double-hotspot on a gas giant ring in the bubble. Lot of carriers there. At least two-thirds of the rocks contain platinum, most in the 20-40% range with occasional 50 and 60+. Also plenty of surface and subsurface deposits, but honestly I ignore them unless they're on a rock I'm also surface mining because surface mining is faster for me. I'm not as fast as the YouTube guys, but I can usually get 192 units in cargo and another 6 in the refinery in about 90 minutes from takeoff to landing at a nearby good (but seldom "best") sell location. I value my time more than an extra 2,500 credits per unit!
The only trouble I have is one that has plagued ED for a long time, but that seems worse than ever for me these days. The yellow-orange hotspots revealed by the DSS never show up any more. It used to be that dropping into regular space and re-entering supercruise would reveal them if they weren't showing, and in extreme cases a log to main menu would always fix it. None of that works anymore for me. On rare occasion the hotspots will flash for a quarter-second when I re-enter supercruise, but immediately disappear. It's not a huge crisis because the hotspots still show up in Nav panel and with markers on the HUD, but I do miss being able to see the EXACT location of most significant overlap between adjacent hotspots.