To build on Johan's comments about spurious marking of published photos as "modified", Adobe has decided not to address one set of causes of this problem.
When a new version of LR adds new develop settings (e.g.
the Guided Upright Tool in LR 2015.6) or changes the internal catalog representation of existing settings (e.g.
brush, linear, and radial masks in LR 11), the catalog upgrade process doesn't change all the photos at once. Rather, as LR re-renders photos, the develop settings will be changed on the fly. When LR eventually changes the internal representation of the settings, it compares the new representation with the old and declares that the photo has "changed" and marks the photo as "modified", needing republishing.
So as you scroll through Library, you might occasionally stumble across a photo whose thumbnail isn't in the preview cache, so LR will re-render it, triggering the "modified". Ditto if you zoom Loupe to 100% and the 100% preview isn't in the cache.
Obviously, this catalog upgrade process, which changes the internal catalog representation, shouldn't trigger a "modified", since by design it's not supposed to change the visual representation of photos. More precisely, LR and Camera Raw provide the ironclad promise that a photo will always render the same visually even after multiple application upgrades -- that's why we shouldn't have to worry about archiving JPEG renderings of our photos.
When the Guided Upright Tool was added in LR 2015.6,
according to employee Rick Spaulding, Adobe explicitly decided not to address the spurious "modifieds":
"Due to it’s obscure nature and that a solid workaround has been discovered we will not be expending any time or resources to fix it. It will only affect those users who are planning to migrate from a version <6.6 to a version >= 6.6 so that number is extremely small."
When the wonderful new Masking Tool was added in LR 11.0, employee
Rikk Flohr queried the product team whether it was desired behavior that published photos would be marked as "modified". There wasn't a follow up with their answer, and the bug report remains in Discussions (i.e. not officially acknowledged and logged in their internal bug tracking).
You could easily work around previous instances of this bug by selecting all the modified photos in a published collection, right-clicking, and doing Mark As Up-To-Date. Unfortunately, that doesn't work with the LR 11 instance -- photos marked as up-to-date will rise from the dead and remark themselves as "modified" sometime later.
The only reliable workaround for LR 11 is to first rebuild all the Library previews (to force a re-rendering) and then republish all the photos spuriously marked as modified (I've got several hundred published collections containing over 15K photos). The Publish Collections command of the
Any Source plugin makes it easier to find and re-publish these photos.
Bug threads:
https://community.adobe.com/t5/ligh...blishing-and-smart-collections/idi-p/12250355
https://community.adobe.com/t5/ligh...otos-to-re-publish-for-no-reason/m-p/12690169