Certified GKH Gardening Expert
Peonies are finicky in their blooming. Any disturbance can make them skip a year. What I think likely happened is that one year the pink bloomed and the white failed to and the next the white bloomed while the pink failed to bloom. I also have a multi-color peony bed, and never know which plants will bloom in any year. Some years they all will go and others only one will open. A million different stressors can cause this.
It may seem like they are sectioned off in particular places, but if they are a well established peony bed, then the tubers have grown into their neighbor's beds. So, where you may have planted white a few years ago, if the pinks are nearby, they have spread into the white and vice versa.
Try to reduce stress to the plants as much as possible. Make sure they are getting enough water and light, that the turbers are not being buried too deeply by mulch, that they have enough nutrients and, importantly, their roots are not being disturbed in any way. this should encourage all colors to bloom at once.