Try to address the concurrency problem

app/models/member.rb

@@ -104,8 +104,12 @@ class Member < ApplicationRecord
   end
 
   def self.regenerate_all_notifications
-    Member.all.each do |member|
-      member.regenerate_notifications
+    ActiveRecord::Base.transaction do
+      ActiveRecord::Base.connection.execute('LOCK members IN ACCESS EXCLUSIVE MODE')
+
+      Member.all.each do |member|
+        member.regenerate_notifications
+      end
     end
   end
 end

app/models/notification.rb

@@ -4,8 +4,12 @@ class Notification < ApplicationRecord
   scope :scheduled_for_today, ->() { where(status: 'scheduled', to_be_sent_on: Date.today) }
 
   def self.send_scheduled_for_today
-    scheduled_for_today.each do |n|
-      n.deliver!
+    ActiveRecord::Base.transaction do
+      ActiveRecord::Base.connection.execute('LOCK members IN ACCESS EXCLUSIVE MODE')
+
+      scheduled_for_today.each do |n|
+        n.deliver!
+      end
     end
   end