Everytime I make the smallest change to a picture in Lightroom (for example adding new keywords) when I goto the 500px module it says "Modified - Need to be Republished".
The "republishing" erases the existing title and captions from the 500px website, leaving me with a bunch of public images on the site that are "Untitled" and have blank captions. Some of the captions were a paragraph or more long but since I wrote them a year or more ago I have no idea what was in there so re-writing them is a pain. If I'm lucky somebody commented on the caption and I can guess what it was about, other times I still have no idea and its like listening to the comments as a one sided conversation.
How can I stop Lightroom from corrupting my 500px data?
Lightroom CC, running on Windows 7 Professional
This is controlled by the publish plugin, which was written by 500px. Usually a publish plugin is designed so that any title or caption in Lightroom will be republished with the picture. If the title or caption is blank in Lightroom, maybe that would erase whatever is on 500px. But as 500px designed the plugin, you would have to ask them how to avoid this happening.
This is a work-by-design: when Lightroom knows that an image has been modified, it notifies the Publishing Service, putting the image in status 'to Re-Publish'.
As John said, then, it is the publishing services to decide what to do. For example, some publishing services do not modify the Title, other do modify the metadata but do not publish the new image (so, the only way to change an image is to delete from the published site and republish). This is decided by the publisher, not by Adobe.
Why don't you simply right-click / option-click on the image marked as "to Re-Publish" and select "Mark as Up-To-Date" ?