ṣ is substituted for the s of nis before the verb tap when meaning is not that of 'repeatedly making red hot'.,
The word आसेवन means doing a thing repeatedly and here it means making it red-hot repeatedly. Thus निष्टपतिः सुवर्णम् = सकृत् अग्निम् स्पर्शयति i.e. he puts the gold into fire only once.
Why do we say when not meaning repeatedly. Observe निस्तपति र्सुवर्ण सुवर्णकारः = पुनः पुनरग्निं स्पर्शयति ॥
In the sentences निष्टप्तं रक्षः, निष्टप्ता अरातयः the change has taken place either as a Vedic irregularity or because the sense of repeatedness is not implied here.,