Rama did take his power from Parashurama but he regained it by bathing in the tirtha of Bhrigu as explained in Vana Parva Section XCIX. Lomasa told Pandavas that by bathing in the tirtha of Bhrigu, he will regain his energy taken away from Duryodhana like how Parashurama regained his might.
And behold there, O Yudhisthira, the tirtha of Bhrigu that is celebrated over the three worlds and adored, O king, by great Rishis. Bathing here, Rama (of Bhrigu's race) regained his might, which had been taken away from him (by Dasaratha's son). Bathing here, O son of Pandu, with thy brothers and Krishna, thou wilt certainly regain that energy of thine that hath been taken away by Duryodhana, even as Rama regained his that had been taken away by Dasaratha's son in hostile encounter."
Parashurama got his powers back which explains why he was still a formidable warrior in Mahabharata.