The Joe Biden Administration didn't continue the Remain in Mexico policy and brought over people to America who they lost contact with after scheduling a court date.
"Remain in Mexico" isn't a law, and in fact the legality of that original Trump policy is still unsettled (but it probably isn't legal without an act of Congress given its clear incompatibility with the INA). The 9th Circuit shut it down within its jurisdiction before the Biden admin did.
