Experienced the same, was charged for one day where the bill shows it consumed 100KB while roaming in Japan.
Complained to Celcom, then took the complain number to CFM website. Within the week Celcom responded that the rebate will be reflected in the next month's bill.

Alot of forumers suggest precaution steps such as:
- turn off mobile data entirely / but roaming enabled (this will allow calls to come in, minus the data)
- manual operator select