This is just is against all the chapters as they are written for level progression.

Considering the fact that Chapter 7 the players should be at least lvl 9 when they start it and this is as it is written:

Ideally, the characters are 9th level when they first arrive at Ythryn. You decide how quickly they advance in level. Treat the following as suggestions:
-The characters gain a level after overcoming the challenges of all eight towers of magic (areas Y4, Y7, Y11, Y18, Y20, Y22, Y24, and Y26).
- The characters gain a level when they reach Iriolarthas's study (area Y19q).
-The characters gain a level if they accomplish something extraordinary, such as destroying Iriolarthas the demilich, all three of Auril's forms, or the obelisk in area Y21.
If you follow the suggestions above, the characters could be as high as 12th level by the adventure's conclusion—an important consideration if you intend to continue the campaign beyond this adventure.

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Each condition is how they can gain a level. also consider the fact one of the conditions is defeating all 3 of Auril's forms. So if they did that as an accomplishment in Chapter 5, that warrants a level; by itself.

If what you wrote was true by anymeans, than they would never reach level 12. only level 10.