I went trough in survival to the end city and never got epic item so I started to check and debug and even on a brand new world it seems like the world tiers don't change. In the attached log you can see in lines this:
Line 757: BlueGhost has made the advancement [World Tier: Haven]
Line 777: Current GenContext: GenContext[tier=haven, luck=0.0, dimension=minecraft:overworld, biome=minecraft:flower_forest, stages=[]]
Line 821: BlueGhost has made the advancement [World Tier: Frontier]
Line 825: Current GenContext: GenContext[tier=haven, luck=2.5, dimension=minecraft:overworld, biome=minecraft:flower_forest, stages=[]] (luck increased here because i equiped armor with it to make the frontier achivement)
Line 849: BlueGhost has reached the goal [World Tier: Ascent]
Line 858: Current GenContext: GenContext[tier=haven, luck=2.5, dimension=minecraft:overworld, biome=minecraft:flower_forest, stages=[]]
latest.log
Using neoforge-21.1.115 and Apotheosis-1.21.1-8.1.3
Also the log is from a new world with a lot of other mods but I also tested it with just Apotheosis mods and the bahavior was the same. That log is also below.
latest2.log
The expected bahavior would be that the tiers would change on there own but they only change when changed manualy by a command.

World Tiers are not expected to change on their own, you have to use the world tier menu to change them, as mentioned in the first advancement.

Would it be considered to "force" the player's tier as a config option?

World Tiers are not expected to change on their own, you have to use the world tier menu to change them, as mentioned in the first advancement.

The first achievement is not given to the player normally, so the player has to go to the achievements tab (after the haven achievement) to see that they have more than they see in the chat. I think that's a bad design. Not everyone looks at them at all, so expecting every player to read descriptions of achievements they didn't see in the chat is too much in my opinion.