I think it depends on how you define trough of sorrow, there are many versions this phenomenon can manifest. In a company I consulted, they had PMF in one segment but the segment was to small to generate quite serious growth - hence they hit a trough / growth stall. While going after a more lucrative segment, competitors started to catch up - diminishing profit from the original segment as well.

Turned out positioning was the problem - they still had PMF in both segments, the issue was that they were not able to communicate what they offered properly, leading to leads choosing competitors instead. Changing their comms put them back on track.