That sounds like the game creates new units based on the BP of the units in full share. Would be the same base issue as upgrades done post creating of the units not feeding into the veterancy bonuses I saw somewhere else.
There is some mod that transfers all your units to one player automatically when you die, that idea probably could be merged into the full share function to allow for a better full share feature solving this issue.
Maybe the Game Team can investigate the viability of my proposal and implement it if feasible.
