So do you suffer with indicators that have a life of their own! Does one stay on while the other one flashes like mad suggesting a blown bulb but all hazards work ok?
Don't pull your hair out there is a solution!
If there's anything I've learned in the short time I've owned a GTA is always always check connections.
In this case. Unplug the indicator relay take it out dismantle it and unless there's any obvious damage, tracking or popped capacitors spray it with contact cleaner. Reassemble and check connectors before replugging. I would then recommend removing all indicators and cleaning contacts with fine wet & dry paper and spray with contact cleaner. Reassemble and all should work fine again.
You may have an issue with the switch but this is less likely. Its either wiring, connectors or earth but the flasher unit isn't earthed directly so I'd plump for connectors or a fried unit if none of this works!