Fix

lib/assigner.rb

@@ -547,8 +547,7 @@ def already_assigned?(assign_to, type, note, status)
   end
 
   def reassign?
-    return false if !@target.is_a?(Topic)
-    Assignment.exists?(topic_id: @target.id, target: @target, active: true)
+    Assignment.exists?(target: @target, active: true)
   end
 
   def no_assignee_change?(assignee)

spec/lib/assigner_spec.rb

@@ -264,6 +264,20 @@ def assigned_to?(assignee)
         expect(second_assign[:success]).to eq(true)
       end
 
+      it "assignments limit doesn't prevent from reassigning a post" do
+        posts = (described_class::ASSIGNMENTS_PER_TOPIC_LIMIT).times.map do
+          Fabricate(:post, topic: topic)
+        end
+
+        posts.each do |post|
+          user = Fabricate(:moderator)
+          described_class.new(post, admin).assign(user)
+        end
+
+        status = described_class.new(posts.first, admin).assign(Fabricate(:moderator))
+        expect(status[:success]).to eq(true)
+      end
+
       context "when 'allow_self_reassign' is false" do
         subject(:assign) do
           assigner.assign(moderator, note: other_note, allow_self_reassign: self_reassign)