CCK text field select list options widget data incorrect when call drupal_execute() more than once

Drupal 6.6 CCK 6.x-2.2

I am creating CCK based content programmatically using drupal_execute().
Everything works fine if only one form submission is made per HTTP request.
If more than one call to drupal_execute() is made then the issue arises.

ISSUE
====
In the code below 2 cck_widget_test content nodes are created.

The field_fuel_type CCK field is a text field with the select list option widget type enabled.
Its allowed values are petrol and diesel.

In the first generated node the resultant field_fuel_type content correctly contains 'petrol'.
In the second (and all subsequent) it contains only the single first character 'p'.

If the field_fuel_type widget type is changed from select list to text then the issue does not occur.

Tracing on hook_form_alter and hook_field for the first node produces:

hook_form_alter => array(1) {
["values"]=>
array(3) {
["title"]=>
string(11) "widget test"
["field_fuel_type"]=>
array(1) {
[0]=>
array(1) {
["value"]=>
string(6) "petrol"
}
}
["op"]=>
string(4) "Save"
}
}

hook_field field_fuel_type for validate => array(1) {
[0]=>
array(1) {
["value"]=>
string(6) "petrol"
}
}

hook_field field_fuel_type for sanitize => array(1) {
[0]=>
array(2) {
["value"]=>
string(6) "petrol"
["safe"]=>
string(6) "petrol"
}
}

Tracing on hook_form_alter and hook_field for the first node produces:

hook_form_alter => array(1) {
["values"]=>
array(3) {
["title"]=>
string(11) "widget test"
["field_fuel_type"]=>
array(1) {
[0]=>
array(1) {
["value"]=>
string(6) "petrol"
}
}
["op"]=>
string(4) "Save"
}
}

hook_field field_fuel_type for sanitize => array(1) {
[0]=>
string(6) "petrol"
}

So it looks as if by the time hook_field is called for the second node things have gone astray.

Any ideas on this?

I will try some exploratory tracing in optionwidgets.module.