A sound change in Albanian?

Usually PIE *s- yields gj in Albanian. I notice that in two words PIE
*s yields sh when followed by a *t

-të gjt --> sht
PIE *swek's --> gjagj --> gjagjtë --> gjashtë

PIE *septm. --> gjta --> gjtatë --> shtatë

Are my linguistic skills improving? Of course the modern
pronunciation of gj may have been different in either phase 2 or phase
3...

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