Checked it on my servers here and amavis selects the correct policy (mailbox has precedence before domain rule). I verified this also in the amavis config file where the order is set correctly in the sql querys:
' FROM spamfilter_users LEFT JOIN spamfilter_policy ON spamfilter_users.policy_id=spamfilter_policy.id'.
' WHERE spamfilter_users.email IN (%k) ORDER BY spamfilter_users.priority DESC';
$sql_select_white_black_list = 'SELECT wb FROM spamfilter_wblist'.
' WHERE (spamfilter_wblist.rid=?) AND (spamfilter_wblist.email IN (%k))' .
' ORDER BY spamfilter_wblist.priority DESC';
Maybe you use a modified amavis configuration file on your system and not the one that is delivered with ISPconfig?