Meta Onsite Result

Usually when I downlevel someone in a coding interview, there's a clear foundations gap between:

• The candidate being able to concisely talk about code at both a high and low level
• The code the candidate typed out

I'd look to see if your recruiter can give you more details on what reads they got.

...this was even after acing both the medium level coding questions with the correct Time & Space complexities

There is usually far more the Meta interviewer expects from you with data structures and algorithms questions than just the optimal solution with good time/space analysis. I made an entire course about it here: Master The Data Structures And Algorithms Interview

Under what circumstances is a candidate typically down-leveled after the onsite interviews at Meta?

At Meta, the LeetCode round is largely pass/no pass but you can infer some leveling signal. The bulk of the leveling signal comes from system design and behavioral. Down-leveling happens when the LeetCode round is healthy and the system design and behavioral answers are showcasing behavior of the level below. For behavioral in particular, they want signal that you can adapt and learn quickly so you can rise up quickly from the downlevel. This is especially relevant for E3/E4 as they are subject to up-or-out.

However, since you're currently Staff and already interviewing for E5, I'm unsure if Meta is able to extend you an E4 offer.

Best of luck!

Unfortunately, it's hard to get feedback from any interview. There's a lot for the company to lose by sharing the feedback with you, and not much to gain.

Instead of focusing too much on what you may have done wrong, I suggest doing a lot more interviews! You only need to one company to give you the level you're targeting.