Once you've added an addon (or resource pack) and created a world, your addon will say goodbye.

When creating a world with one or several add-ons or resource packs, all or some of the content is randomly disabled. This forces the player to return to the menu and re-enable the content. The occurrence conditions seem very random, and I have tried to reproduce this several times with different texture packs, add-ons, and multiple others, but have not been able to record it.