After killing specific waves of enemies during the finale event there are multiple delays of 1-4 mins for new enemies to start spawning in which is a huge disruption to the flow of combat.
This is the main reason i almost never play the horn of magnus.
I keep witnessing this on righteous stand as well, globs of slave rats get stuck on the ceilings. At least you can kill them to keep the event moving though. This intermission stops the game dead as you say.