in the final arena of Righteous Stand (where Sigmar Statue is located) I encountered an issue twice.
After the first wave of enemies attack, nothing spawns for a very long time.
Then a Special spawns and another wave comes in. Then another long wait and the same thing repeats but it takes a very long time to complete the map.
Both of the games I encountered this was deed.
In the first one I was solo with bots and in the second I had another friend with me.
Both of the deeds had Seek & Destroy (more specials) property on them, if it helps.
Another thing I noticed in the second game, when there were no enemies visible inside the temple, friend of mine threw a Javelin at a pillar where Skavenslaves climb down and as soon as the javelin hit the pillar dead skaven bodies became visible and dropped down.
I tried to explain it the best way I could, I hope it’s helpful.