@1110101001 I guess the reason is the following note in the documentation of InputAutoReplacements: "In expression input, automatic replacements can be performed only on strings of characters that correspond to complete input tokens." Compare the following two box structures:

MathLink`CallFrontEnd[
 FrontEnd`UndocumentedTestFEParserPacket["a->", True]]
(* {BoxData[RowBox[{"a", "->"}]], StandardForm} *)

MathLink`CallFrontEnd[
 FrontEnd`UndocumentedTestFEParserPacket["a[[", True]]
(* {BoxData[RowBox[{"a", "[", "["}]], StandardForm} *)

As you can see, [[ is not a single token at this stage, while -> is

(more generally, [[ is never a single token - even a[[b]] is represented by the box structure RowBox[{"a", "[", RowBox[{"[", "b", "]"}], "]"}])

@AaronStevens Have you checked the tag [boundary-condition-at-infinity]?: mathematica.stackexchange.com/questions/tagged/…

Here is someone saying that he cannot accept answers to his old questions. Is this legit? I have never heard of it before, and cannot find any information.