Rare as it is—and I’d argue it’s not as rare as some people think, having experience three avionics GPS loss-of-signal events in the past two years (in different aircraft)—they’d be nuts not to have considered that risk in the design. I can’t find the AFM supplement for Safe Return but to a first approximation, it wouldn’t be out of the question for it to go into a heading and altitude hold until it reaquires.