HTML input array name coming through as a string - php

Question

I have an input checkbox field as follows:
<input type="checkbox" name="group_items[<?=$item_id?>][<?=$group_id?>]" value="1" />
There are several of these in a matrix grid. On submission, it should post something like this:
(array) Array
(
[1] => Array
(
[1] => 1
[3] => 1
[5] => 1
[9] => 1
[13] => 1
[15] => 1
)
[2] => Array
(
[1] => 1
[3] => 1
[5] => 1
[9] => 1
[13] => 1
[15] => 1
)
[3] => Array
(
[2] => 1
[4] => 1
[8] => 1
[10] => 1
[14] => 1
)
[4] => Array
(
[2] => 1
[4] => 1
[8] => 1
[10] => 1
[14] => 1
)
)
Most of the time, this seems to be okay.
I then run this:
if(!empty($_POST['group_items']))
{
foreach($_POST['group_items'] as $item_id => $item_groups)
{
$group_ids = array_keys($item_groups);
foreach($group_ids as $group_id)
{
// do something
}
}
}
However, on occassion, I get this error:
Uncaught PHP Error: array_keys() expects parameter 1 to be array, string given
It's as though it's posting this:
(array) Array
(
[1] => (string)
)
I am well aware that I can perform an is_array check on $item_groups, however I need to understand how this can even be a string instead of an array.
I am also aware that I could change the values from 1 to be what the key is currently set to, and have a zero-based index, like this:
(array) Array
(
[1] => Array
(
[0] => 1
[1] => 3
[2] => 5
[3] => 9
[4] => 13
[5] => 15
)
)
However this would not do anything to resolve the issue of the value of [1] being a string instead of an array.
I'm also unable to log out the post data as a client has lost data due to this error and logging the post to see what it contains would risk them losing more data. This is also why an is_array check is not the solution as it would mean the posted data has still somehow been lost and doesn't solve the actual cause.
I have checked the server limits for the maximum number of fields that can be posted and it is high enough; I can submit hundreds of checkboxes and they come through fine. This also seems to be an intermittent error and can work as expected deveral times, and then cause the error the next.
Does anybody have any suggestions on a cause?
Edit: Have now got a dump of the data when it errors.
(array) Array
(
[0] => 1
)