The IPA is based on the amount you earn so if your earnings are reduced then you IPA should also be reduced. This should be the case whether you are forced to reduce your hours, your job changes to part-time or you decide to go on strike.
If this is only for one month and depending upon how much your wage is reduced the OR may opt to keep the IPA the same and expect you to make up for the reduction of the excess money once the IPA is paid. As you only pay between 50% and 70% of your DI you should have some money left over.