In 2d9f0217ec904a1952a1ada2136502eefbca562e (RUBY-2220) the MapReduce builder was changed to specify read preference externally to the command payload itself, so that it could be correctly passed to pre-3.6 servers. Unfortunately this fix broke read preference passing from Mongoid which relied on the MR builder in driver including read preference in the command payload. This affects Mongoid 7.4-stable going back to at least 7.2.0.

On Mongoid master this was fixed with the following change:

index afe68fc7a..964537005 100644
--- a/lib/mongoid/contextual/map_reduce.rb
+++ b/lib/mongoid/contextual/map_reduce.rb
@@ -147,7 +147,7 @@ module Mongoid
       def raw
         validate_out!
         cmd = command
-        opts = { read: cmd.delete(:read) } if cmd[:read]
+        opts = { read: criteria.options.fetch(:read) } if criteria.options[:read]
         @map_reduce.database.command(cmd, (opts || {}).merge(session: _session)).first
       end
       alias :results :raw

... in e2c854a9a90f9f19ef6077b4b185aeb319e4b9d7.