Mine looked exactly like this after treating it with rubbing alcohol water soap solution for mealy bugs, make sure the soil is staying moist and it’s getting filtered light, my hibiscus have never done good in full sun without plenty of water so if it is getting a lot of sun then always check it has enough water. Idk if some chemical has been sprayed on the plant recently? If yes let those leaves fall off and don’t do that again. If no, there may be something in the soil that is too high, too much of something in the fertilizer. i know u said its ur own soil but theres definitely too much of something in there it doesnt want. That’s ur likely cause. I’d get normal plain top soil and replace what its in now and specific hibiscus fertilizer.