Your theory is almost certainly correct. The Mendix Windows service is running under that deactivated AD account. When the service writes files to the network drive, it does so under whatever identity the Windows service is configured to run as. If that account got deactivated, the behavior can get weird in ways that dont always break everything at once, depending on cached credentials and token lifetimes.
Check the Windows service on the server, look at the "Log On" tab, and see what account its running under. If its the former developers account, switch it to a proper service account with the right NTFS permissions on that L:\ drive. This has nothing to do with OIDC or the Azure client secret, and nothing to do with Docmosis either. Those are application-level concerns, the file writing to a network share is purely an OS-level thing determined by the service identity. The timing with the secret renewal is just a coincidence since it happened around the same time the developer left.