Hello,

First, I'm using mongoid 7.0.7, but I've tested this on mongoid 7.1.1 and the results are the same.
This bug was not present in mongoid 6.2.1 .

About the bug :
When we declare an embeds_one/embeds_many field on a document, and we specify a class_name with a "::" root namespace (ie "::Test::Foobar"), the parent of this embedded document will not be set, and will return nil.

What I found is that the method determine_inverses in Mongoid::Association::Embedded::EmbeddedIn/Mongoid::Association::Embedded::EmbedsMany/Mongoid::Association::Embedded::EmbedsOne have this line : rel.relation_class_name == inverse_class_name

rel.relation_class_name return the value given by the option class_name, the string with the root namespace "::", but inverse_class_name return the class name without any leading "::", because it's simply a call to `name` on the class of the parent.

Here's some code showing this behaviour :

#### Test with embeds_one
class Test
 include Mongoid::Document

embeds_one :test_foobar, class_name: "::Test::Foobar"
end

class Test::Foobar
 include Mongoid::Document

embedded_in :test_parent, class_name: "::Test" # Doesn't work with this line
 #embedded_in :test_parent, class_name: "Test" # Work with this line
end

t=Test.new
t.test_foobar = Test::Foobar.new
# Should NOT be nil
t.test_foobar.test_parent


#### Test with embeds_many
class Test
 include Mongoid::Document

embeds_many :test_foobars, class_name: "::Test::Foobar"
end

class Test::Foobar
 include Mongoid::Document

embedded_in :test_parent, class_name: "::Test" # Doesn't work with this line
 #embedded_in :test_parent, class_name: "Test" # Work with this line
end
t=Test.new
f=t.test_foobars.new
# Should NOT be nil
f.test_parent