The bug affected one of my friends, so I did the most logical thing that is there to be done...

I vibe-coded wrote a script to fetch the list of participants who got their rating changed while being master+!

import requests
def get_high_rated_handles(contest_id, min_rating):
    url = f"https://codeforces.me/api/contest.ratingChanges?contestId={contest_id}"
    response = requests.get(url).json()
    if response["status"] != "OK":
        raise Exception("Failed to fetch rating changes.")
    result = response["result"]
    qualified_handles = [r["handle"] for r in result if r["oldRating"] >= min_rating]
    return qualified_handles

handles = get_high_rated_handles(contest_id=2155, min_rating=2100)
for handle in handles:
    print(handle)

list of affected participants:

I think this happens when you register for the contest before rating changes of previous round get applied.. Anyway, it's kinda unfair for them to get a negative delta (some got a huge negative delta) while supposedly being unrated.

also I ran the script on a couple more past div2's, but apparently this issue didn't happen in them (maybe it's a new bug, or the system already handles the issue after some time..).