There was a Eurocontrol advisory in 2014 regarding more than a dozen reported false RA's associated with hybrid TCAS installed on Airbus single aisle models. I believe the TCAS were all vers 7.1.
There may be more conclusive explanations of the events by now, but it was suspected by the TCAS manufacturer that when the hybrid unit, under certain relative position geometries, transitioned from ADS-B to TCAS ranging, the result created a range rate that tripped the RA.
This excerpt from the advisory may help:
"The common factor in these events is that the RA receiving aircraft is fitted with TCAS II version 7.1 with hybrid surveillance function. The RAs are triggered when two aircraft two aircraft are crossing at the same level or are in vertical convergence but where conditions for RA generation are not met and ATC standard horizontal separation is assured. Typically, the unexpected RA is generated on the ‘front’ aircraft only against an aircraft that is crossing between 5 to 7 NM behind it. Only A319 and A320 aircraft were receiving false RAs but the anomaly may affect other aircraft types as well.
In all reported cases, there has been no erosion of horizontal separation or possible risk of collision between the two aircraft at the time when the ‘front’ aircraft in the encounter received an RA; hence, from a pilot and controller perspective the RAs have been unexpected. Nevertheless, the pilots have, quite properly, followed the RA instructions. Whilst this is correct, from a traffic management viewpoint the manoeuvres disrupt the flow and increases cockpit and ATC workload; they may also precipitate follow-on conflicts."