When is a secondary DNS server used?
I googled this article which explains newer MacOS DNS search order. And this one which explains how to tweak it to obtain results that you desire.
Though the general idea is that it was never intended (in any OS) that first server is the one used and the second one is a backup. ( Even on windows, if first server for some reason doesn't answers very quickly, the second one will be queried.) It's wiser to regard server query order as unspecified.
The general idea of a secondary DNS server was that in case the primary DNS server doesn't reply (e.g. it is offline, unreachable, restarting, etc.), the system can fall back to a secondary one, so it won't be unable to resolve DNS names during that time. Doesn't reply means "no reply at all", it will not ask the secondary when the primary one said that a name is unknown. Answering that a name is unknown is a reply.
The problem here is that DNS uses UDP and UDP is connectionless. So if a DNS server is offline, the system won't notice that other by not receiving a reply from it. As an UDP packet may as well get lost and the round-trip time (RTT) is unknown, it will have to resend the request a couple of times, every time waiting for several seconds, before it finally gets to the conclusion that this server is dead. This means it can take up to an entire minute and above to resolve a DNS name if the first DNS server dies.
As that seems unacceptable, different operating system developed different strategies to handle this in a better way. As both DNS servers are supposed to deliver the same result for the same domain (if not, your setup is actually flawed as the secondary should be a 1-to-1 replacement for the primary one), it shouldn't matter which one is being used. Some systems may send a request to the primary one but if no reply comes back within a few seconds, they don't resend to it but first try the secondary one (then they resend to the primary one and so on). Some may also query both at once, make the faster one win and then keep using that one for a while (until they start another race to see if it is still the faster one). Some may also prefer the primary one but do some kind of load balancing and switch to the secondary one if more than a certain amount of queries are currently pending on the primary one. Some will just alternate between them as a poor man's load balancing. All of this is actually allowed.
In your case, though, I'm afraid something is wrong with your primary server as by default, macOS will only use the primary one. If it constantly falls back to the secondary one, it may consider the primary one to be too slow. Every time that happens, the secondary server becomes the primary one, see this older knowlebase article. This cnet article explained how this can be disabled but I'm not sure this is still possible in current systems. I wasn't able to find any reference on this but IIRC from the very back of my head, Apple once mentioned on a WWDC that they are now more aggressive at DNS querying and may even try to contact multiple DNS servers at once with the fastest one winning in some cases but I might be wrong on this (maybe this was iOS only or so).