Your letters do not go anywhere when an inmate is released. Everything stored in your InmateAid account stays there until you decide to deactivate the account yourself. Release does not affect what is saved on the platform.
If you are not seeing letters that should be in your account, it is most likely a navigation issue rather than missing content. Log back into your account and look for the inbox or received mail section. Letters your inmate sent back through InmateAid are stored there and should be accessible the same way they always were.
If you have gone through the account carefully and something still appears to be missing, reach out to InmateAid's support team directly. They can pull up your account on their end and confirm that everything is accounted for. In situations like this they will typically send you a direct email walking you through exactly where your letters are stored so nothing feels lost.
The good news on top of all of this is that your person is home. Whatever is in that account is a record of the time you both got through, and it is yours to keep for as long as you want it.