It's possibly a question of different chart pictures from NOAA, for different purposes.
If you download your chart from this page Chart Downloader for NOAA RNC®
and then extract the picture from the kap file with imgkap by
imgkap 18773_1.KAP 18773_1.png
and view this picture, you will find the same "problem" as in OpenCPN's view.
My conclusion is that NOAA does no longer print the update info on the charts
, but the info is contained in the kap files text header.
The only problem I can find with OpenCPN
is that the NTM line in the kap file is not processed. This is the line for 18773_1
OpenCPN tells the update date, but not to which NTM.