QRs used to state that pay ceased on being missing and I think only some pay was restored when you were reported as being a POW. there was then a presumption that food, subsistence and pocket money pay would be issued by the detaining power.
That worked to a point when the detaining power abided by the rules.
In GW 1 the MIA pay stopped immediately however I believe there was a rapid revision of this when it was realise dwhat hardship it caused and compounded for a family.