Grouping is unbound after restart

There is a problem with grouping devices. I’m running two Volumio sets (one is on the PC, second is a Raspberry). When I group them it works perfectly, but after rebooting the grouping is gone and I have to group them again. Is there a way to keep them grouped all the time?

What causes unbound to start in a bad state?

What do you mean by bad state? These are fresh Volumio installs, the master is on a PC laptop and the client is on a Raspberry. After rebooting the master the group bindings are gone and I have to make group again, this is all I know.