What could cause this ceiling fan and light issue?
It looks like the wiring is wrong. The fan and light are interconnected somehow and should be separate. As it was working, it's something you have altered or it occurred itself.

Separate all the wires. See which wires go the light. Connect a lampholder with a bulb to the incoming white wire and one of the other two not counting the green/bare wire

Try your dimmer, and see if it dims the test bulb. If so that non white wire is the one that controls the light. If not change to the other non white wire. That should determine the right wire for the light.

The one that did not control the light is the one that does control the fan, that when connected, should work from the speed controller. The fan and light should now both work but independently.