When exporting a large amount of photos (10,000+) using "Export as Catalog" the parent catalog loses all of its standard previews.

I have had this issue since Lightroom 13.5 and was hoping it would eventually get fixed but it still hasn’t.

On a Mac Studio M1 Ultra (MacOS 26.5.2 but it has happened on OS 15 onwards) when using “Export As Catalog” where I select “Include available previews” and export a relatively large amount of photos, the parent catalog will lose all of its previews requiring a deletion and regeneration of all standard previews.

The exact number of photos being exported that causes the issue is hard to pin down but over 25,000-30,000 will almost always cause it. I can also make the issue appear by exporting a bunch of small catalogs that eventually total up to a larger number of photos (ie. exporting 4 catalogs of 6000 photos).

During the export of the catalog, I will know the bug has occurred because it will skip the “copying previews” step of the “export as catalog” process. Upon browsing the parent catalog, all photos will show as grey boxes. Before closing Lightroom if I try generating previews it will say “No standard previews were built, all standard previews were up to date”. I have to close and re-open Lightroom to get it to generate the standard previews. I can leave the Previews.lrdata in place or delete it before preview regeneration with no apparent difference in outcome. If I don’t delete the Previews.lrdata, its file size also doesn’t change by any significant amount after regeneration which makes me believe it finds the original previews during regeneration and overwrites them.

This issue has occurred for two years now. It happens on brand new fresh catalogs (with a sufficient number of photos) and older catalogs that I have used for a long time.

I’d be able to replicate on demand for an engineer to see.

Hoping we can get this resolved as the 6+ hours a week I have to wait for regenerating previews is driving me nuts.