Microstation project explorer and fields placed in each sheets titleblock is being used to place and update sheet numbers.
If the numbers are updated and then printed from an existing ips, then the sheet number change is not reflected. If the change is made and then the dgn is opened closed and then printed from the existing ips the change is reflected.
I have seen this issue before and set the variable IPLOT_ENABLE_MS_READWRITE=TRUE, but it is not fixing the problem.
Check the model properties in each file for the "Update Fields Automatically" checkbox. After checking this box, close the files. InterPlot Organizer will now be able to recognize the changes without having to first open and close the dgn.