P: Photos are marked for republish even though there is no change.

Adobe... are you reading posts in this forum? I'm thinking and hoping you do. Can you confirm that this bug is on your list of issues to be addressed please?

Note that rebuilding previews and then setting the entire collection as published might (or might not) work, in any case it will trigger all files to be backed-up again (and that's a few 100 Gb for me...). Maybe a possible temporary workaround but not the solution we need obviously 

Posted in "The Lightroom Queen" forum and now posting here.

I've been using Jeffrey Friedl's great "Folder-Publisher" add-in for years now, as it allows me to create an identical parallel folder structure with all my photo's exported as JPGs with all my edits applied.
There's one problem however which, as Jeffrey also told me, was raised for many years and several times already to Adobe, but seems to have remained unaddressed since the perpetual LR version era years back.
As I can't formulate it better than Jeffrey does in one of his blogs, here's his description: "Sometimes photos just spontaneously move from “Published” to “Modified Photos to Re-Publish”, regardless of the settings in “Metadata that Triggers a Republish” and regardless of what's actually been changed."
As you can imagine, this largely defeats the whole publishing purpose. I have 22K+ photos and if I make a few changes and want to update my JPG folders, instead of a quick publish, sometimes 800 extra pictures I never touched (but maybe viewed? I'm not entirely sure) are added. So now the publish process takes a much longer time (plus my system backups later include these uselessly updated photos as well).

Please Adobe, I'm seeing that many users have been reporting this for many years now. This completely defeats the who Publishing functionality. Can you please investigate, debug and fix this problem and make us all happy? 

Sounds like you may have bigger issues. The problem is, you shouldn't have to re-render anything. There should be a simple "flag" which indicates if a photo (regardless of its preview) has not been modified since the last time it was published.

That flag is broken and has been for years.. At this point it's just plain negligence on Adobe's part.

Re-rending the photos solved that porblem! But, now my catalog keeps getting corrupted for some reason: probably unrelated.

On 8/13 through the insistence of  Adobe support on the phone with me, I removed my Flickr link service to LRC and reinitiated it.  I am pleased to say to date I haven't had any random republishing events in my Publish Services for Flickr. (Mac Computer)

@johnrellisquite honestly, I'm surprised that this problem hasn't reappeared for me 😅 but so far, so good. 🤞

I'm wondering whether it might have to do something with the preview cache itself. As I mentioned, LRC decided to nuke my entire preview cache after a system migration.

@aliceingdid you already try letting Lightroom Classic re-render previews for all photos in the entire Flickr collection?

Cmd/Ctrl+A, menu Library, Previews -> Render Standard-Sized Previews.

I was really hoping there was a solution to this. I have over 20,000 photos in my Flickr collection. Ever since I upgraded to Lightroom 11, every time I go to publish new photos or republish select old photos Lightroom starts pulling up random photos from my catalog by the 1000s. Republishing them doesn't help and telling Lightroom the photos are up to date only helps temporarily. Even marking them up to date is like whack a mole because Lightroom keeps pulling up more photos by the 100s as fast as I can mark them. It's hugely frustrating 😞

"I'm mentioning this because having LR render Standard previews for all published photos in a collection seems to update whatever metadata change triggers this."

Glad that's working for you.  Forcing LR to re-render previews hasn't helped me and others in this thread, unfortunately.