If the device is enrolled in work management, add one more line to the list and put it at the top: none of this may be yours to fix.
A configuration profile can restrict search and Siri behaviour, and a managed app can be prevented from indexing content at all. From the user's side it looks exactly like a broken index that no amount of restarting repairs, because it is working precisely as configured.
Settings, General, VPN & Device Management will tell you whether a profile is installed. If there is one, the fastest path is asking whoever manages it rather than trying anything on this list.