Courtesy of HoogaBoogaMooga on Reddit, an exploit I didn't know existed that has never been fixed.
Yeah this has been an exploit since the beginning of the game
If you can go to service by requirement -> to volunteer army-> service by requirement again you'll gain all your manpower (% of population) back again
Jesus what a fucking flawed game! I reverted back to HOI3 (with blackice mod) after about 2 months of playing this shitpile. After 2 years and x DLC's you'd think they would sort their shit!