Yellowing leaves on hibiscus
Hibiscus leaves can yellow for several reasons. Older leaves yellow and drop and can be normal. Yellow leaves can indicate the soil is may be too wet or too dry. Keep the soil evenly moist and make sure the pot has a hole for good drainage. Check for insect feeding on the undersides of leaves or webbing.
How long has the plant been in the container. The plant may be root bound if you see roots coming from the drainage hole or surface roots on the soil surface. It may need repotting.
How often are you fertilizing. They are heavy feeders.
Use at half the label recommended strength every 2-3 weeks when placed outside for the summer.
Take a look at the link from U of Minnesota for care and reasons for yellowing leaves. https://extension.umn.edu/house-plants/hibiscus#fertilizing-1440064