Chief Pentagon spokesman Jonathan Hoffman has said that a lot of TBI symptoms are late developing and manifest themselves over a period of time.
The Centers for Disease Control and Prevention says on its website that some symptoms of concussions and other traumatic brain injury can appear right away, but other symptoms may not be noticed for days or months after the injury.
https://www.cnbc.com/2020/01/29/more-us-service-members-diagnosed-with-brain-injury-after-iran-missile-attack.html