While I was trying to make a datapack for adding a few more boss, miniboss and armour variants for my modpack I noticed that in each dimension the Apothic Boss Summoner is not spawning all mobs from the pool listed on github - it is also not spawning any of my custom bosses or gear sets. List of bosses spawning below:

• Overworld: Skeletons, Strays, Husks & Zombies
• Nether: Piglins & Zombified Piglins
• End: Endermen, Endermites & Shulkers

I have tested this in an instance with just apotheosis & its dependencies to make sure it's not being caused by another mod with the same results.

Nevermind, I answered my own question as it does indeed work.

I see, I was not aware of that. Will this work with additional mobs from datapacks etc when the datapack is functioning properly?

The boss weights vary by world tier and can be restricted by other context. You can view the actual spawn pools for the current context by running /apoth debug weights invaders