Don't clear start at task flag until all hosts are advanced

Clearing the flag after the first host was advanced caused all other
hosts to not advance at all.

Fixes #13864

lib/ansible/executor/play_iterator.py

@@ -137,6 +137,7 @@ class PlayIterator:
                 self._blocks.append(new_block)
 
         self._host_states = {}
+        start_at_matched = False
         for host in inventory.get_hosts(self._play.hosts):
             self._host_states[host.name] = HostState(blocks=self._blocks)
             # if the host's name is in the variable manager's fact cache, then set
@@ -152,16 +153,20 @@ class PlayIterator:
                         break
                     if task.name == play_context.start_at_task or fnmatch.fnmatch(task.name, play_context.start_at_task) or \
                        task.get_name() == play_context.start_at_task or fnmatch.fnmatch(task.get_name(), play_context.start_at_task):
-                         # we have our match, so clear the start_at_task field on the
-                         # play context to flag that we've started at a task (and future
-                         # plays won't try to advance)
-                         play_context.start_at_task = None
+                        start_at_matched = True
                         break
                     else:
                         self.get_next_task_for_host(host)
+
                 # finally, reset the host's state to ITERATING_SETUP
                 self._host_states[host.name].run_state = self.ITERATING_SETUP
 
+        if start_at_matched:
+            # we have our match, so clear the start_at_task field on the
+            # play context to flag that we've started at a task (and future
+            # plays won't try to advance)
+            play_context.start_at_task = None
+
         # Extend the play handlers list to include the handlers defined in roles
         self._play.handlers.extend(play.compile_roles_handlers())