/dhadh / धध्

dhA धा + any hard /Git tense
→ **dadhA- दधाॱ
→ dadh- दध् by znAbhyastayor... znAbhyastayor...
→ !**dhadh- धध् by dadhastathozca

As this dhadh धध्, by an exception, does not trigger jhaSastathordh..., it ends up making the same forms as if it were dhad धद् — **dhattas धत्तस्.

What exception? Why doesn't it trigger jhaSastathordh...?

Because of the adhas अधस् word at the end of jhaSastathordh....

Uh? Tricky! Why didn't /pANini just add dhat धत् to sUtra dadhastathozca instead of adding adhas अधस् to jhaSastathordh...?

Uh, figuring that one out should be your homework.