Automatic recomposition of lost or missing -HDR.dng and -Pano.dng files.

It depends what you did to them before. If you accepted the defaults, you can just select those raw files and bake new pano or HDR files. But if you chose merge settings that were not default, no, those settings are lost.

Are the missing DNGs still shown in the catalogue, and with Develop adjustments or Library metadata? If so, you could merge 1232-1234 again. You will then get a DSC1234-Pano-2.dng, so two DNGs for the image - one new, one old and missing. You can then sync the adjustments/metadata to the new one, and remove the old one from your catalogue.

https://forums.adobe.com/people/john+beardsworth  wrote

If so, you could merge 1232-1234 again. You will then get a DSC1234-Pano-2.dng, so two DNGs for the image - one new, one old and missing. You can then sync the adjustments/metadata to the new one, and remove the old one from your catalogue.

You can't sync or copy the develop settings from a MISSING file.  You would have to go to the operating system and copy or rename the new DNG DSC1234-Pano-2.dng to DSC1234-Pano.dng and then restart restart Lightroom and the file will no longer be MISSSING and should reflect all the develop settings.  Then remove the MISSING DSC1234-Pano-2.dng from Lightroom.

Yes, you're right. It may be a bug as Photo > Develop Settings > Copy and Sync Settings are both enabled but do nothing, and one can certainly sync the settings via a plugin.

So yes, use the OS to rename the file. I was hoping to avoid that complication.

If you notice, the SYNC button in this situation is DISABLED so the menus should be also.   The data is apparently there for missing photos so this operation could be allowed as a feature.

The data is certainly there - I could copy everything from a missing file to another using my Syncomatic plugin.

Equally, the menus are enabled so it's the Sync button that's wrong. It's more that the UI is inconsistent. Some elements imply it should be possible, and it is possible to sync if you have smart previews defined, but other UI elements imply it's not possible. My guess is that it's more likely to get fixed if it's treated as a bug

I agree.  If the menus are enabled than the feature should be enabled.  Beings Syncomatic works the sync in Lightroom should be possible.