Well it depends what you mean by "not as expected".
RSSI can be misleading because a strong signal is a higher number. So a -65dBm RSSI is many times stronger than -90. The -100db is where the "noise floor" starts. We want to be above that so the radio has a chance to still distinguish the useful signal from the noise. If your module is well within range, it might yield a very strong signal even at the lowest transmit power setting. At that point ATC can't make it transmit a weaker signal (ie "closer to the target" of -90 or whatever threshold).
So here's what you can expect.
If the RSSI reaches -90 (or whatever threshold is set) at a transmit power above 0, then ATC will keep it at that level to meet or exceed the RSSI threshold.
If the RSSI is already stronger than the threshold at power level 0, ATC can't ramp power down any more. It means you have a very strong signal at the lowest transmit power. Good thing, and ideal case.
The DS18B20 is a sleepy example, it transmits then sleeps. It does not actively listen for packets from the Gateway example. For that, use the Node example instead, which listens and does an sendACK() when a packet is randomly received from the gateway.