Meta E4 full loop experience, rejected

I recently interviewed at Meta for an infrastructure software engineer role. I thought the interview went well; however, I was ultimately rejected without feedback.

Phone Screen: I answered both questions within 25 minutes, and the interview concluded early.

1. Custom sort string
2. Convert Binary Tree to Doubly Linked List

Onsite:

Interview 1: Behavioral This round may have been where I fell short. I only dedicated a couple of hours to preparation and should have focused more on Meta's values. The questions were standard.

Interview 2: System Design Design a gaming leaderboard. I met all requirements, and the interviewer seemed pleased.

Interview 3: Coding

1. Copy a linked list with random pointer. I struggled initially but produced a working solution within 15 minutes.
2. Vertical traversal of a binary tree. Follow-up questions involved solutions that avoided sorting an array of columns at the end.

Interview 4: Coding

1. Valid palindrome (ignoring whitespace, case, and non-alphanumeric characters). This was an easy question, but I rushed and introduced a minor bug, which I quickly resolved.
2. Sparse vector dot product. This problem was vaguely described, and I identified two approaches. I coded two complete solutions: one using a hashmap for each vector and another using an array instead of a map (requiring the second vector to be a regular vector).