I asked the Canadian dealer, Solaire Canada. He asked FLARM Technology. He just got back: "We have confirmed with the manufacturer of our GPS receiver modules (u-blox) that they handle the GPS week rollover event on April 6th 2019 correctly. We thus do not expect any issues on that date. This applies to all Classic and PowerFLARM devices, as well as OEM devices using a u-blox GPS receiver." So, no worries for PowerFLARMs and rollover.
Cheers