Yes, you're correct.
That’s why I asked whether you have come across any other documentation. The issue is jurisdiction (in personam jurisdiction - - jurisdiction over the person). As an active duty service member, the service has jurisdiction to try you. As a retiree, the service has jurisdiction to try you for, example, offenses committed while on active duty - - prior to retirement. The service is required to place a person on legal hold, i.e., hold off on issuing a DD-214 and activating retirement, prior to imposition of trial / sentence. What I’ve seen happen in the past is a miscommunication between the command and retirement folks, which is often times corrected last minute. For example, a command may start proceedings, but the retirement branch, in the interim, approves a retirement date. The command sends a legal hold, but the retirement branch fails to process, and retires the person. The line of cases dealing with this have favored the government in explaining that the retirement was defective, therefore, the government retained jurisdiction over the servicemember. See, e.g., United States v. Harmon, 63 M.J. 98 (C.A.A.F. 2006). Additionally, there are cases where the retirement branch entered an “eligible retirement date” before or after a legal hold was made; however, the legal hold will trump that date, even if the person’s record reads that they’ve been retired as of that date. It’s akin to an admin error.
If you were validly retired prior to trial, you have a legal challenge against your conviction. If, however, a hold was requested, but was not acted upon by the retirement branch, then you likely do not have a legal challenge. It all hinges on whether a legal hold was validly issued.
If you do not have additional documentation, but wish to sue, you should make a request for all documentation - - which you’re legally entitled to have. If there’s no evidence of a legal hold, there likely was none, and you have strong legal grounds to challenge your conviction.