>> However, the power supply uses a more complicated design to provide electrical isolation between the spacecraft and the clock. I'm not sure, though, why isolation was necessary.
Because soviet spacecraft, as with all spacecraft at that time, were designed to survive all sorts of failures. The clock is an absolutely essential part of the spacecraft. It needs to keep running even when everything else is failing, especially when everything else is failing. A space clock needs to be both integrated into a dozen other systems, and able to cut itself from those systems when necessary. Cosmonauts in a failing capsule, waiting to fire the return burn necessary to get home, won't be looking out the window. They will be looking at that clock.
Because soviet spacecraft, as with all spacecraft at that time, were designed to survive all sorts of failures. The clock is an absolutely essential part of the spacecraft. It needs to keep running even when everything else is failing, especially when everything else is failing. A space clock needs to be both integrated into a dozen other systems, and able to cut itself from those systems when necessary. Cosmonauts in a failing capsule, waiting to fire the return burn necessary to get home, won't be looking out the window. They will be looking at that clock.