OnDataBuilderComplete is only triggered only when the active playmode script has changed, and you enter playmode (line 69 of AddressablesBuildScriptHooks.cs).
Is there a particular reason why you would want an event for when a build is finished? You could create a new custom build script that inherits from the default one, and after base.BuildData do any custom post-build actions.
I was trying to implement a moddable items system. I have other data (.json) files that gets created with metadata about the assets(mods). I load these files first, which has all the info I need about an asset without having to touch the Addressables until they are needed. I was trying to get a callback when build is modified so that the potential modder does not have to manually save it. More of a convenience for the modder that might have limited knowledge of unity/Addressables.