Short answer. Only if the order was still a lawful one connected to your work. The Labor Code's disobedience ground reaches lawful orders in connection with the job, so a genuinely unrelated demand is weak footing for dismissal — but a job description is not the outer limit of what may lawfully be asked.
What the law says
willful disobedience by the employee of the lawful orders of his employer or representative in connection with his work
Labor Code, Article 282 — Just Causes For Dismissal. Read the full provision →
'In connection with his work' is broader than your job title
The phrase the article uses is in connection with his work, not "within his job description". Employers are generally allowed to direct how work is carried out, including reasonable adjustments to duties, coverage during absences, and tasks incidental to the role even where nobody wrote them down. So the fact that an instruction is not listed in the document you signed on your first day does not by itself take it outside the article. The connection the law asks about is to your work, broadly understood.
Where the connection genuinely breaks
There is a real limit, though. An instruction with no relation to the business at all — personal errands for a manager, work at a superior's home, or tasks that amount to a different job entirely — is much harder to bring within the article. The same is true of a demand that is unsafe or that you are not licensed or qualified to perform. In those situations the employer is not exercising its right to direct your work; refusing is not the wilful defiance of a lawful work order the ground is built around.
A reassignment is a different question from a single task
Be careful to separate two things that feel alike. A one-off request outside your usual duties raises the disobedience question directly. A permanent shift of your functions raises a different issue altogether — whether the change is a legitimate exercise of management prerogative or a demotion dressed up as reassignment, particularly where pay, rank, or benefits move with it. If what happened to you is really the second, the analysis does not start with whether you refused.
How to refuse without handing them the case
A flat refusal is the version that fits the article best, so avoid giving them one. Ask what the task involves, say in writing why you believe it falls outside your work or beyond your qualifications, and ask for the instruction to be confirmed by someone with authority. That record — the request, your written objection, the reply — is what decides these disputes later. Keep your job description, appointment papers, and any notice to explain, and take advice before the disciplinary hearing.