High FC can cause false high pH readings. I don't know if it also can generate a false high TA.

Meanwhile, when you say you adjust TA, are you doing it by our ratchet-down, aeration method? You lower pH with acid to about 7.0. Then aerate to raise pH without raising TA. Repeat when pH is back around 7.5 until TA is where you want it.

If you aren't doing it this way, you aren't lowering TA...so I'm wondering if that's it.

Carl