13 covers on my 3* bullseye but showing 158 as max level. I have him 4/5/4.
The max level button shows you how far you can get with your existing ISO and a single (massive) purchase of additional ISO.
Nothing is wrong here.
Hrm. My max level button shows level 166 for mine. What does your level cap say?
This “feature” trips up most people the first time they see it. The level up button shows the maximum level you can reach with your current iso8 plus the biggest iso8 purchase you can make, just like Dayv said.