FECiDA Desk Grow Lights for Indoor Plants, Bonsai, Seed Starting, Table Top Grow Light UV-IR Full Spectrum, 25 Watt Power Draw, Bright Enough LED Growing Lamp, 16"-24" Height Adjustable

I’ll guess it’s lack of light, overwatering, and lower humidity than it’s used to. Having it inside a heated low light apartment is probably drying out the air. Check the soil to make sure it’s coarse bonsai with little organics instead of the mud commonly used for retail store plants - mud soils are notoriously for over soaking roots while at the same time having pockets of impermeable hardening clay. Zooming up close on the picture it looks like it has too much organic/potting style soil. Strong full spectrum grow light, not those cheap blue-red wands. https://www.amazon.com/FECiDA-Spectrum-Houseplant-Adjustable-Function/dp/B0BLCRXY54