The bug_actiongroup has the following code
$f_assign = gpc_getint( 'assign' );
if ( ( NEW == $t_bug_data->status )
&& (ON == config_get( 'auto_set_status_to_assigned' ) ) ){ #by pluntke
$t_assign_status = config_get( 'bug_assigned_status' );

The check of $t_bugdata->status to be NEW is missing in some other scripts, where status is set to bug_assignedstatus.
In my installation I have changed enum like : new, assigned, in_process, ....

Problem is, when an issue is already in_process it gets "downgraded" to be only assigned when assigning to another developer.
In fact the code above should check if the issue state is below than bug_assigned_status and only then change status.