The problem has now spread to the three adjacent plants we have in large pots, which have plenty of sunlight.
Cancerous Like Growths on the Leaves of the Hibiscus Plants
Certified GKH Gardening Expert
It sounds like they have galls. These can be unsightly, but are typically harmless to the plant. They can get unsightly though. Some galls are caused by a fungus and will spread, which sounds like what is happening here, so the leaves will need to be removed and destroyed (including any foliage on the soil around the plants). You can also treat them with neem oil to help control the problem.