Until now I don't find any simple working solution for a dynamic include in Entity Framwework Core 2.1. Below is a simple case where I put the Include in another line that doesn't work.
string[] includes = new[] { "navigationprop" };
var query = _dbContext.Set<T>();
query.Include(includes[0]);
This work:
var query = _dbContext.Set<T>().Include(includes[0]);
In this answer is a similar issue but i think it's too over.