Ultralow addresses changes from time to time

I still don't get it. I have this issue where I periodically have to switch from ultralow1 and ultralow2 in my private DNS settings. I am doing this because I have always been routed by nextDNS to the slowest ultralow server, not the fastest. Problem is, sometimes ultralow1 showing lightnode-xxx, sometimes ultralow1 shows premiumrdp-xxx and vice-versa. And sometimes even if I use id.dns2.nextdns.io for ultralow2, I am still routed to ultralow 1 (slowest)