You can receive healthcare reimbursements in both countries, but you need to follow specific administrative procedures to do so.
The country where you work is your reference country in terms of social security because contributions are usually (but not always!) deducted there directly from your salary.
Starting work abroad while living in Belgium? Belgium is no longer competent with regard to social security, and you will therefore have to follow some procedures and notably contact the relevant organisations.
Here are the procedures you need to follow:
If you work in Luxembourg, document S1 is replaced by document BL1
Are you in the opposite situation? Find out what procedures you need to follow if you work in Belgium and live abroad.
All the procedures you need to follow with the mutual insurance company if you live in Belgium and work abroad.