Do not remove ENTRY_SECRET during apply difference

This probably occurs with bot accounts only.
It is strange that the qts is used by bots but is missing from the
results of getDifference. This would need more investigation, but
it might just be the way this thing works.

Closes #3873.

telethon/_updates/messagebox.py

@@ -302,17 +302,23 @@ class MessageBox:
     #
     # Should be called right after login if [`MessageBox::new`] was used, otherwise undesirable
     # updates will be fetched.
-    def set_state(self, state):
+    def set_state(self, state, reset=True):
         deadline = next_updates_deadline()
 
+        # TODO there have been multiple reports where `end_get_diff(ENTRY_SECRET)` is used during `apply_difference`,
+        # but because the `map` no longer contains `ENTRY_SECRET`, `reset_deadline` fails. AFAIK that can only happen
+        # here (not sure why Telegram's update difference state lacks the new `qts`; this needs investigating).
+        # If the difference state does not contain info about `pts` or `qts`, ignore it and don't remove the state.
+        # This issue probably occurs with bot accounts only but I have not verified it.
+
         if state.pts != NO_SEQ:
             self.map[ENTRY_ACCOUNT] = State(pts=state.pts, deadline=deadline)
-        else:
+        elif reset:
             self.map.pop(ENTRY_ACCOUNT, None)
 
         if state.qts != NO_SEQ:
             self.map[ENTRY_SECRET] = State(pts=state.qts, deadline=deadline)
-        else:
+        elif reset:
             self.map.pop(ENTRY_SECRET, None)
 
         self.date = state.date
@@ -586,7 +592,7 @@ class MessageBox:
         chat_hashes,
     ):
         state = getattr(diff, 'intermediate_state', None) or diff.state
-        self.set_state(state)
+        self.set_state(state, reset=False)
 
         # diff.other_updates can contain things like UpdateChannelTooLong and UpdateNewChannelMessage.
         # We need to process those as if they were socket updates to discard any we have already handled.