The problem region is the region where Maxwell calculates the field solution. It is just enough that Region size (solution space) be chosen such that the respective region walls are not close to field sources(current or permanent magnets). This allows us to assume the field is a tangent at the region limits. The region must be given sufficient room for fringing.
For example, if the magnetic fields are well-contained, as in motor, the padding can be 20-50%.However, if the magnetic fields are not well-contained, such as two parallel current-carrying conductors in air, the padding may need to be 1000% or more.
*If the field results change significantly when the region size is changed, the region is not large enough.
If you have this create region option in Maxwell 15 use this option and pad all the directions to around 1000%. then you should not see this issue.
You can check by increasing padding all the way to 2000% - you should not see any difference in result.