Hibiscus: Is this a fungus or seasonal leaf drop?, 5 by LAS14

LAS14 wrote:
We have 4 indoor hibiscus plants. Back in September one of them developed yellowing leaves with black spots on the back and other disfigurement. Spraying with a fungicide seemed to stop it. Now, in late December, all four plants have yellowing leaves, but no other disfigurement. Do hibiscus seasonally drop their leaves? We've had them for a long time, and I seem to recall yellowing in the past that didn't concern me, but after the fungal experience I'm afraid not to spray. But I don't want to as it involves moving heavy plants into the middle of the room on a piece of plastic.

The first 3 pictures are the scruffy ones from September (actually early October by the time I took the pictures), and the last 2 are from December.

TIA
LAS