fix #53020

Author: i1oveMyse1f

pandas/_libs/tslibs/timestamps.pyx

@@ -1026,7 +1026,7 @@ cdef class _Timestamp(ABCTimestamp):
             base1, base2 = base, ""
 
         if timespec == "nanoseconds" or (timespec == "auto" and self.nanosecond):
-            if self.microsecond:
+            if self.microsecond or timespec == "nanoseconds":
                 base1 += f"{self.nanosecond:03d}"
             else:
                 base1 += f".{self.nanosecond:09d}"