Lightroom Desktop: Slideshow video exportation: does not take edits into account

I'm talking about a lot of images, like for a time lapse with over 500+ images

Granted, with the 1080p 16:9 limitation, LR is maybe not suited for this kind of exportation, but it's nevertheless not the expected behavior for rendering a slideshow ;-)

A typical scenario:
Syncing all the images with the desired edits, wait until synced (of course) and then try to export through the Slideshow module.

A more advanced workflow involves "reading metadata from file" instead of syncing edits from one picture, but the end result is the same.

For what I understood, LR seems to render correctly only the images that have been rendered (not per se 1:1 previews) in the Library view. I mean, when you sync edits, and view an image, it takes some time (sometimes a fraction of second) to display the synced edits. If there are a lot of images, a lot of them are not displayed and thus not rendered.
Rendering the 1:1 preview is just a (long) workaround to force rendering. This is, however, overkill for the 1080p limit of LR.

This is the test I ran:

Create Collection of 485 Images all in color and synced
Convert all Images to Black and White
Allow edits to sync to cloud
Go to Slideshow and Export collection as video

Do I have that correct or did I miss something?

No, I'm talking about LR Classic only, so no cloud sync involved.
I'm talking about edit sync in the develop module:

Revision:

Create Collection of 485 Images all in color
Select one image
Convert to Black and White
Sync to other 484 images
Go to Slideshow and Export collection as video

Is that right?

Right ;-)

Ok. I was able to reproduce with these steps. 

If I go to Grid before Slideshow and scroll through all the images until I see the visual update of the file, it appears to generate correctly. Does that match your experience?

Yes.

But if you DON’T scroll through the grid view and don’t update the previews and generate the video, LR uses the « previous » previews instead of the right ones.

Ok. I've duplicated what you are seeing and logged a bug. Thanks for the report and the instructions. 

Thanks Rikk!