Losing more than 50% of the FC in a day with a very high CYA level means you've got more than just loss from sunlight. You could check the overnight chlorine loss, but I suspect you've got nascent algae growth even if it isn't visible -- again, if the CYA reading is truly as high as reported.

Usually even in very hot sunny weather the chlorine loss will be around 15% in a day when the CYA is 100 ppm or so. At 60-80 ppm it may be around 20-25% FC drop while at 40-50 ppm it may be up to 50% and at 20-30 ppm it could be up to 60%. Higher CYA levels have lower chlorine loss rates even when proportionately keeping the FC higher. It may have to do with some direct CYA shielding effect protecting lower depths from sunlight.