On versions < 7 I used this gem: https://github.com/ElMassimo/mongoid_includes which is doing its job quite good, but unfortunately is not compatible with version >=7.

But I believe that I came up using that gem because of the same issue - it looked like nested eager loadings were still ending up with N+1's.

So here's a minimalistic example gist to demonstrate this issue:
https://gist.github.com/FunkyloverOne/2024bb96c3db8be301a8d10e1abd7968