Sounds like the weirdness that goes on with TCAS traffic displays on the 747-400...
TCAS displays will not appear on the ND unless the IRS's have been aligned. However, the TCAS traffic displays don't always disappear when you switch the IRS's off (on some aircraft). It's as if the displays need to see a valid IRS signal one time only.
The results vary with which ND mode you are in (VOR/ILS/MAP), TCAS Computer types, what reference is being used (Track or Heading), etc. The traffic will disappear completely however if you switch the TCAS off and back on again without IRS's.
Just out of interest, Synthetic, what reference does your Map use... HDG UP or TRK UP?
Regards.
Q.