Per Incident 2644048: Allow ResExtract to show results for each sub-plan inside a master-plan.
I can informed that this behaviour is currently as designed. It is not possible for ResExtract to extract results for each plan that is a sub-plan of a master plan because what ResExtract does is that it takes a res file and puts the contained data to a txt file. That means this will happen once a res file is available - so it cannot happen earlier. If a master plan is executed, a res file will not be available during the execution of the sub-plans in the master plan, therefore, no data will be extracted until the end of the master plan. Given that you execute a master plan file you will get a result for the master plan file and when this is available the resextract will also be run.
Therefore, if you would like to have such a behaviour that ResExtract extracts results of sub-plans executed via a master plan, you could either create your own logging mechanism and write to a file using a custom helper function. However, you will not be able to log builtin messages/errors/warnings to such a custom log file. Alternatively, you could log this request as an enhancement request to be considered in for a future release of Silk Test in the online feature request page