ODATA filtering "$top=50" causing unexplained repetition

Question

I'm working on an API and the object I'm dealing with has many, many records. I'm accessing it via this URL:

{{api_root}}/Objects?$top=50

For time keeping, I'm limiting the query to 50. However, I recently added a list of strings IDs and have ran into this issue: the list, obtained via these lines:

.ForMember(
    d => d.FooIds,
    o => o.MapFrom(
        s => s.FooSys.Where(q => q.Fee != null)
                     .Select(q => q.Fee.Id)))

gives me a repeating list of length 30 of one unique ID, and only returns a second object, again with a repeating list of 30 unique IDs, like this:

"data": {
    "list": [
      {
        "ObjectId": "ID1",
        FeeIds": [
          "UniqueID", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", "UniqueID Copy"
        ],
        ---more object data---
      },
      {
        "ObjectId": "ID2",
        FeeIds": [

          "UniqueID", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", 
          "UniqueID Copy", "UniqueID Copy", "UniqueID Copy", "UniqueID Copy"
        ],
        ---more object data---
      }
    ]

However, if I get a single object by its ID, only one UniqueID appears. And if I shorten my top: "$top=10", then I get 10 copies of FeeIds instead of 30.

Has anyone seen this type of behavior before? I'm very confused!