"Warning" is the most lenient remedial measure for disciplining any misconduct on the part of an employee. "Leave" is a statutory or contractual welfare benefit of employment subject to certain restrictions. However, outright denial of any leave applied for without any valid reason, such as exigencies of work, is a bad HR practice when there is sufficient leave credit of the employee and the purpose of leave is reasonable.
Sanctioning leave for an employee is only an occasional affair. Therefore, the decision to sanction or refuse leave should be based on the occasion and not on any behavioral issues of the employee. If the earlier warning was for issues like frequent leave-taking, unauthorized absence, etc., which are detrimental to the organization's work, the employer's hesitation can be justified. No hard and fast rules can be laid down, and only the employer can be the best judge of the situation.