While doing a 5* Dungeon in U8, after doing 4/5 of the dungeon i ended up figuring out that one boss was missing. After the timer from the 5* ended, the last boss finally spawned.

Occuring how much?

Very Rare
Rare
Sometime
Often
All the time

See video:
https://youtu.be/hxO3ngrMgU0

I also previously encountered a 5* dungeon reading " */4 dungeons" Instead of */5 dungeons"