There are a few different possible causes. Could be that both DRAMs U18 and U19 have failed, which can sometimes happen. Or as mentioned above, half of the data bus bridge has a problem; likely U12 or U13 missing one of the control signals. Might be a defective U12 or U13, which is less likely.
Would be easy to see what's going on if you had an oscilloscope, so without one my suggested approach would be:
1. Carefully inspect the PCB for any physical damage, scratched tracks, etc.
2. Inspect the IC sockets for contamination or damage of any contacts, especially U2 and U5. If in doubt, replace the socket.
3. Replace U18 only, run DiagROM again and see if data bits 8-11 are now working, if so, replace U19 as well and that should fix it.
4. Else you could try replacing octal buffer ICs U12 and U13.
5. If that doesn't fix it, you likely have a connection problem to U12/U13. Which would be a few seconds to find with an ocilloscope, or a few hours to find with a continuity tester. Though you'll probably struggle to correctly measure an open circuit if it's caused by a contact issue in the PLCC84 socket (U2). So you could try replacing the U2 socket if you don't find an obvious connection/continuity problem.
