REST API for achievements gets different results vs Server-side APIs

We are creating a project where we need to use different REST APIs, one of which is the user achievements. We discover a strange behavior when calling this particular one. We have this example where a user has 3 achievements and we have a call like this:

api.ashx/v2/achievements/user/336065.json?PageSize=10&PageIndex=2

as a result, we get the full list of achievements:

Now, if I do the same with the Server-side API:

#set($pagedListResponse = $core_v2_userAchievement.List("%{ PageIndex = 2, PageSize = 10, UserId = 336065 }"))

I get what I think is expected, no achievements because I'm asking for an index that is not available:

Another example, if I call api.ashx/v2/achievements/user/336065.json?PageSize=10&PageIndex=0

I don't get any results:

Doing the same with the Server-side API:

#set($pagedListResponse = $core_v2_userAchievement.List("%{ PageIndex = 0, PageSize = 10, UserId = 336065 }"))

I get the expected results, the complete list of achievements:

We are running version 12.1.4.25477