If I correctly understand your problem, your converters are connected in parallel for powering their own load and you are measuring the differential potential between the ground of your non isolated converter and the primary ground of the isolated converter ?

How do you do your measurement ? With an oscilloscope or with a multimeter ? I think you are trying to measure the average voltage across the capacitor between the primary and the secondary of your transformer of your isolated converter. 

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[![enter image description here][1]][1]

As you can see you are trying to measure the voltage across your transformer isolation, ie the potential difference between the primary ground and the secondary ground. 

As @hacktastical said, the isolation is not perfect. Nevertheless I think your measure is bad as you are paralleling a high impedance (the one of your multimeter) and the high isolation of your transformer. Your measure modify the system. It is an "intrusive" measurement. And the voltage value given by the multimeter is not correct as you need to measure an impedance negligeable with respect to the internal impedance of your multimeter for having correct measurement.  So it is complicated to say (at least to me) what is the meaning of your voltage measurement. 


  [1]: https://i.sstatic.net/2XD0e.png