The active infinitive is uniform (-re from -se by rhotacism) across the regular Latin conjugations, but the passive one is not: the third conjugation loses the consonant. We have amare/amari, habere/haberi, audire/audiri, and trahere/trahi. By analogy one would expect traheri instead of trahi. Why is the third conjugation passive infinitive ending like this, while other regular conjugations have -re/-ri for active/passive? Is there a reason, context, or anything that would help understand this apparent loss of a consonant?
This is a subquestion of this older question. The broader question remains unanswered, but perhaps this is more tractable.