Digging out the RTL SDR (and waiting for Ubuntu to install in a VM because I've temporarily broken my gnuradio install under MacOS again), it's as I suspected: the new "broken" temperature sensor is broadcasting off-frequency. The three dots under the center line in this waterfall display are the three sensors that behave themselves, broadcasting at 433.92 MHz. The one off to the right at about 434.4MHz is the rogue.

Given that 434.4 is still in the ISM unlicensed band, I'm planning on keeping the sensor and working around the problem in software.