I want to know if the 20% holdout is totally random without considering class imbalance issue or 20% out of every class? If is the former I am afraid of minorities might be totally missing from the holdout; if is the latter then the situation of the imbalanced minority will be even worse. Anyway, if holdout can be exported then I can find out the answer by myself but I can't find how to export the holdout.
Whether the holdout would consider the classes or not depends on the partitioning chosen. If the partition type is Random, then it would not consider the classes and if there are very few minorities it is possible that the holdout would not have any. If you use stratified partitioning then the holdout will have a stratified sample of the majority/minority class.