I'm having a similar problem with most of my plants. Similar conditions to yours under leds except temps are between 12 to 16 °C and no LFS in the mix (just as a top dressing).
I was told by an experienced ceph grower that my plants are dormant and its normal for them to lose most or all foliage while in this phase. They should rebound with vigorous growth once temps increase in my case.
In your case, I'm not so sure this is what's happening since your temps and photo period are not low enough to have induced dormancy IMHO. I would lower the humidity to 40-50%, as established cephs do not require greater humidity, which in fact can cause fungus or rot issues. I would also carefully repot as Willy said into a well draining mix (2:2:1 peat/perlite/silica sand), deep pot (>13 cm tall) and keep the substrate moist but not saturated. Good luck.