Preserve freq in TimedeltaIndex subtraction

Author: Chenghaoxiang

pandas/core/arrays/datetimelike.py

@@ -1089,6 +1089,12 @@ def _get_arithmetic_result_freq(self, other) -> BaseOffset | None:
             # e.g. TestTimedelta64ArithmeticUnsorted::test_timedelta
             # Day is unambiguously 24h
             return self.freq
+        elif (
+            lib.is_np_dtype(self.dtype, "M")
+            and isinstance(other, Timestamp)
+            and self.tz == other.tz
+        ):
+            return self.freq
 
         return None