SequenceCases does something odd with RuleDelayed arguments

Question

I have a list of stuff.

list = {a, b, b, a, b, a, a, a, b, a};

I want to split it up into subsequences where a single a is followed by zero or more bs. It's a perfect job for SequenceCases:

SequenceCases[list, {a, b ...}]
(* {{a, b, b}, {a, b}, {a}, {a}, {a, b}, {a}} *)

Now I can use a RuleDelayed argument to do something to each subsequence.

SequenceCases[list, seq : {a, b ...} :> f[seq]]
(* {f[{a, b, b}], f[{a, b}], f[{a}], f[{a}], f[{a, b}], f[{a}]} *)

Let's use this to consecutively number the subsequences.

count = 1;
SequenceCases[list, seq : {a, b ...} :> (count++ -> seq)]
(* {1 -> {a, b, b}, 4 -> {a, b}, 6 -> {a}, 7 -> {a}, 8 -> {a, b}, 10 -> {a}} *)

Wait, what? Maybe Sow/Reap can shed some light:

count = 1;
Last@Reap[
  SequenceCases[list, seq : {a, b ...} :> Sow[seq, count++]],
  _, Rule]

(* {1 -> {{a, b, b}}, 2 -> {{a, b}}, 3 -> {{a}}, 4 -> {{a, b}}, 
    5 -> {{a}}, 6 -> {{a}}, 7 -> {{a}}, 8 -> {{a, b}}, 9 -> {{a}}, 
    10 -> {{a}}}*)

For some reason it's evaluating the right-hand side of that rule for each subsequence that matches at all, but only keeping the ones that are the longest. There's nothing in the documentation that specifies what SequenceCases will do if a RuleDelayed argument has side effects, but this behavior is still quite surprising and seems undesirable.

(There is, at least, an easy work-around with Map[Replace[rule]], but it's likely to be verbose and repetitive.)

Answer 1

count = 1;
SequenceCases[list, seq : {a, b ...} :> (count++ -> seq), Overlaps -> All]

{1 -> {a, b, b}, 2 -> {a, b}, 3 -> {a},
4 -> {a, b}, 5 -> {a},
6 -> {a},
7 -> {a},
8 -> {a, b}, 9 -> {a},
10 -> {a}}

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