@Gblenn I teach daily, days and nights so my occurrences are likely more visible. Typically, it fails over to Tier 2 and often is unnoticable. (I find out later looking at logs.)
However, on this latest instance, the Tier 1 was flapping and zoom reported my connection was unstable. Students reported poor audio and video. It was only resolved by changing the backup to Tier 1 and restarting zoom. Later that night my main connection stabilized. so naturally I'm thinking it is trying to switch back as soon as Tier 1 is available. I will test to verify.