Quick question regarding act rank triangle for Immortal+. I've only been playing for a couple months, and have just now gotten to Immortal (and subsequently been getting a lot of last act immortals in my games), but some of how the act rank...

yesterday we were 3 stacking with my friends. we're all plat so when we saw on the loading screen that 2 of the enemies were bronze last act and one was iron, we were already kinda sus of something cause this was a gold/plat lobby. i think...