oj: Inconsistency with JSON.generate in context of a Rails app

In a freshly generated Rails app (created by just running rails new my_app), with only the addition of the oj gem to the Gemfile, assuming that we have a class like:

class Foo
  def initialize
    @x = 123
  end
end

Then, Oj.mimic_JSON() (or, equivalently, Oj.optimize_rails()) fails to mimic the serialization behavior. In a rails console:

foo = Foo.new

JSON.generate(foo)
# => "\"#<Foo:0x00007fc9e0f294f8>\""

Oj.mimic_JSON()
JSON.generate(foo)
# => "{\"x\":123}"

As seen here, Oj serializes the object by serializing its instance variables (probably due to calling foo.as_json internally), differently from JSON which just prints the object as a string.

Note that in Rails using the default JSON, the behavior of object.to_json and JSON.generate(object) differs in that only object.to_json relies on calling object.as_json, but with Oj.mimic_JSON this difference is lost:

class X
  def as_json(arg)
    { abc: 123 }
  end
end

x = X.new

# In a Rails app, before requiring Oj:

x.to_json
# => "{\"abc\":123}"

JSON.generate(x)
# => "\"#<X:0x00007fc8a52ca4c8>\""

# When requiring Oj and mimicking JSON:

require 'oj'
Oj.mimic_JSON()

x.to_json
# => "{\"abc\":123}"

JSON.generate(x)
# => "{\"abc\":123}"

In other words, to correctly mimic the behavior of the JSON gem in a Rails app, Oj should call object.as_json when calling object.to_json, but should NOT call object.as_json when calling JSON.generate(object).

I am running this code in Ruby 2.6.6 and Rails 6.1.3.1, not sure if results vary by version. In a plain Ruby project (not requiring Rails) this problem does not seem to occur, and JSON.generate behaves like the JSON gem version when serializing instances of custom classes. Conversely, if require 'rails' is added before calling Oj.mimic_JSON(), the issue occurs.