Pinterest onsite went well, still got rejected

Result: Rejected Number of Rounds: 5

For some background, I'm 6 YOE, currently at a mid size fintech company but fully remote work is basically the only thing I care about right now, my partner and I are planning to have a kid next year and the WLB reputation Pinterest has was a huge draw for me.

Phone screen was the escape room game problem, players moving between rooms and getting a leaderboard by room and entry time. Took me a bit to get the getTop(k) part efficient but I got there.

Onsite was 3 coding rounds, one was reconstruct itinerary style, another was a sparse matrix storage question, third one had a follow up that tripped me up on edge cases. System design was the catalog update problem, propagating changes across 500M records, we talked a lot about partitioning and retry logic which I felt good about. Behavioral was short and pretty standard, maybe 20 minutes.

Recruiter said HC needed another week and then I just got the rejection with no real feedback. Honestly thought the design round carried me. Anyone else get further along and still get cut like this, or was it probably the third coding round that did me in?