Robert, the ATU is working as designed. The ATU, when enabled is in the RX and TX signals paths. So changing any antenna in any slice will put the ATU into bypass mode and require a re-tune. Granted on RX this really doesn't make a lot of difference, especially in the same band, but for TX, it makes can make a significant difference.
Once we add the automatic / memory feature for the ATU, this behavior will be mitigated as if you have previously tuned it, the LC combination for that antenna port/frequency combination will be in a memory and will automatically be selected.