Deleting of the data from the performance archive also deletes the original (planned) activity data.

Yes, it is expected behavior.

Entering actual data one may change estimates of remaining duration, volume, etc. If something was wrong in performance archive  (in other case why do you want to delete this record?) then it does not make sense automatically restore deleted data. Planned or remaining data may be restored manually or using formulas when current version is compared with the previous or with baseline.