I purchased Owari Satsumas on two occasions from a local orchard with the same results. The fruit were not bad but did not have much taste. I made the purchases in mid and late December. The trees are 6 years old and really look good.
Other than a magnesium deficiency, what would cause this bland tasting fruit?

Satsumas get better as they get older....the Japanes waon't even eat them until tthey are at least 6 years old...maybe older. My satsuma, now 12 feet tall or better, had bland and tasteless fruit until 2005. It was bland until it was about 5 years old.....the older the wood that it blooms on, the better the fruit.