Total items incorrect

Problem/Motivation

I have an odd issue. I have pager serializer in use on multiple views, the total_items on one of them is not working correctly. Here is the pager result for page 1 of the view where total is incorrect:

"pager": {
  "current_page": 1,
  "total_items": 21,
  "total_pages": 3,
  "items_per_page": 10
}

The actual total is 47, but on each page the total_items changes, e.g. page 3 indicates there are 41 total_items (total_pages is also incorrect and changes per page).

Here is an example where the total items are correct (same site, different View)

"pager": {
  "current_page": 1,
  "total_items": "26",
  "total_pages": 3,
  "items_per_page": 10
}

Notice that total_items here is a string... total_items and total_pages are consistent and correct across page numbers for this view.

I can typically figure this kind of thing out on my own, but this one has me pretty stumped. Any ideas on directions to explore would be greatly appreciated.

Additional notes:
- the fact that the first total_items is an int and the second is a string made think we could be changing the value in a hook somewhere, but I can't find anything like that in code (though I am not even sure what that hook would look like and we have a couple hundred thousand lines of custom code, so... )

- both views use a custom entity as base entity with multiple relationships and contextual filters

Steps to reproduce

unknown