When you use TTL flash (automatically metered flash), you can get great images – I use TTL all the time. Like in this image of Anastasia:
But sometimes, oooh, it goes wrong and the image goes too dark. Like here:
I’ll tell you what. Your camera’s evaluative/3D Color Matrix metering tries to expose well, and to avoid over-exposed areas.
And that watch is reflecting the flash. So it would be over-exposed. So the camera tells the flash to fire at lower power- to avoid that. Hence, the rest of the image is underexposed.
Simple, once you know: in TTL flash images, avoid reflective surfaces like the watch!