I tried doing some searching to see if I could figure this out on my own, but I guess I couldn't quite figure out the right search terms, or maybe no one else has run into it, but it seems like Undo doesn't always work. For example:
Sometimes, I will copy settings from one image and paste them over another image, usually an image I've done no editing to at all. For one reason or another, I'll decide that I don't like it and I will undo it with Ctrl+Z to take it back to default as it was SOOC. After doing so, in the History panel, the "Paste Settings" history item will disappear and it will go back to the previous History item, which would be the "Imported (dd/mm/yyyy hh:mm:ss)" history item and SHOULD just be default pre-edit settings, but the appearance of the image retains the pasted settings, and all sliders, including crop settings remain in the position of the pasted settings even though I've technically undone them. Further pressing of Ctrl+Z changes nothing. It doesn't happen all the time, but I've noticed it happens every now and then and it's bugging me not knowing why.
I know that you can select an image and go to Settings > Reset All Settings to set everything back to default, so it's not a huge issue, but it concerns me that undo doesn't always seem to function as designed and I was just curious to know if anyone else has run into this and/or whether anyone might know what would cause this?
Without knowing everything that you are copying and pasting from one image to the next, it's difficult to really assess what is happening. But I'm thinking it's possible that when you use the undo command that Lightroom is "possibly" canceling just the last adjustment though was made rather than all adjustments that you pasted. If that last adjustment was something subtle then it may not be noticeable. I could be wrong.
For the specific scenario that prompted me to post today, in the image I'm copying the settings from (I'll call it the source image), I've only made changes in the Basic panel and in the Tone Curve panel. In the "Copy Settings" window, I've enabled everything except for the "Local Adjustments" and "Crop" categories. In the image I copied the settings to (destination image), no changes had been made to the image at all. Everything was default, so after pasting, the only thing to undo would have been the paste, and in most cases, when I undo that, it goes back to the full default settings and sets the history panel so that it's just the "Import (Date)" history line, except this time. This time the "Paste Settings" history was removed so that the only history item was "Import (Date)" but the sliders/curve remained in the same position as the pasted settings instead of reverting to defaults.
After reading your response, for funsies, I copied the same settings again from the source image and re-pasted them on destination image that won't seem to undo just to see if maybe your suggestion of a minor change was the case. I kept an eye on the basic and curves panels and none of the settings updated. And since none of them updated, it didn't create a "Paste Settings" history for this test either.
Just to further test it, I decided to make a tiny change to a slider in the source image settings, then copy the updated settings over to the destination image. After pasting, the slider in the destination image did update to reflect the change I made in the source image and a "Paste Settings" history item was created for it since there actually was a change to the settings. When I press undo again, it undoes my small change test and takes it back to the "Import (Date)" history that seems to have been corrupted/overwritten by the initial paste settings.
Apologies for the novel! It is quite the perplexing issue, though.
Unfortunately, I can't do a good test of this with this particular image since after I pressed undo, I noticed it wouldn't undo all the way and fiddled around with it trying to figure out why it wouldn't undo, which "overwrote" the redo option that would have redone the initial paste settings and potentially allow me to click the previous edit history step. It's also an issue that seems to be sporadic, so I can't quite reproduce it on another image at will. I can say that clicking the previous step on other images reflects the appropriate settings for that history step.
Though, I did test to see what would happen if I tried to paste more settings and click the previous history step. Just to set the stage, at this point, there is only one single history for the image, which is the "Import (Date)" edit history, but with sliders/tone curve that match the pasted settings, even though they shouldn't. If I paste new settings, or if I make a change that creates a new history step, clicking the previous step takes me back to the seemingly corrupted/overwritten "Import (Date)" history step that contains settings it shouldn't have.
I'll have to keep an eye on it to see if it happens with another image and see if clicking the previous step reverts it back for that image.