“Optional-TTL” generally means the camera detected an accessory (external) flash rather than the built-in flash, while still using TTL/through-the-lens flash metering to control exposure. In other words, it usually indicates normal TTL-style operation with the external unit.

With a compatible flash, the camera meters the flash through the lens and adjusts output automatically. Some flashes also allow flash exposure compensation, which changes flash output while still working within TTL control.

So in your setup, that metadata label is most likely just identifying that TTL metering is being used with an optional external flash, not signaling a problem.