Pi-hole not answering queries when one of two upstream dns servers is down

I recently had an outage on one of my pi-holes. Pi-hole didn't answer queries "just in time", so all queries have timeouted starting around 2:10 pm and ending around 2:40 pm.
From both upstream dns upstream servers that I'm using I saw on my dnsping running every five minutes, that one of them (the first one) was unavailable, but the other/second one was working and answering normally. Adding another dns upstream server temporary made it work again.

:question: So, why wasn't my Pi-hole able to answer my dns requests anymore?

I'm running:

I really would like to upload some screenshots (PNG, about 43 to 126 KByte) to show you some details, but "Sorry, there was an error uploading that file" :man_shrugging: