Soldiers designated with Captive, Missing, or Missing in Action (MIA) status are entitled to receive the pay and allowances to which entitled when the status began or to which the soldiers later become entitled.

Retired Army Reserve Soldiers are not eligible for POW / MIA entitlements. POW / MIA status is designated to soldiers on active duty who are determined to be prisoners of war, missing, missing in action, interned in a foreign country, beleaguered or besieged by a hostile force, or detained in a foreign country.

VA American Former Prisoners of War website:
https://www.benefits.va.gov/persona/veteran-pow.asp

VA POW Fact Sheet:
https://www.benefits.va.gov/BENEFITS/factsheets/serviceconnected/formerpow.pdf

DoD 7000.14-R DoD Financial Management Regulation - Volume 7A: Military Pay Policy - Active Duty and Reserve Pay Chapters 24, 35, 36, 37 and 44:
https://comptroller.defense.gov/Portals/45/documents/fmr/Volume_07a.pdf