this was bugging me a bit, so I experimented.
apparently you can only enter 50 custom properties AT A TIME in the file -> properties dialog. once you enter the first 50, hit ok and come back for more.
if you used a Design Table, I would expect the limit to be 255 - one for each column excel lets you have (leaving one for the config name)
I haven't tried entering more than ~20 though.
Thanks for clearing that up. That's an odd feature.
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