I thought of that, but yesterday I did a shorter time-lapse and only some of my pictures were synced and some not when my final video was exported. That is what made me wonder if/why I had to do them a page at a time.
What you see is entirely normal, apart from this.
Lightroom typically only builds/rebuilds library previews when the image is brought into view, either into the Library grid or the Filmstrip. You can see this most clearly if you "hide" the previews file (e.g. by renaming it), start Lightroom and it will create a new initially empty previews folder and starts to populate it....but only for images brought into view. Page down in the grid and you'll see grey thumbnails, the three dots, and the thumbnails gradually appearing.
So in this scenario I think the same process applies. To test, I just created a new empty catalog and imported 3000 images, and then built standard previews for them all. I then selected the first image, went into Develop, did Ctrl-A to select all 3000, turned on AutoSync, then hit V to turn the first image to B&W. AutoSync then kicked in and started to apply the B&W change to all 3000. I waited for the progress bar at top left to finish, then I went into the Grid view in Library. I waited until all thumbnails in view (using an 8x4 grid, so 32 images in view) were updated, then I initiated an export of all the images.
I waited until about 200 images had been exported before cancelling the Export, checked in Explorer and saw that all 200 had been exported as B&W images, then back in Lightroom I paged down to reveal the next 32 thumbnails....which were of course still in colour initially but quickly changed to B&W as the previews were updated.
All as normal as I expected, but your experience seems to be a little different as you say that some exports still showed the pre-change position. So I have to ask: are you sure you waited until the sync progress bar completed before you initiated the export?
EDIT: Just for completeness, I redid the export but this time let it finish all 3000 images. On inspection post export all 3000 exported jpegs were indeed B&W, and subsequent scrolling through the grid revealed that all the thumbnails (apart from the less than 200 that I had previously viewed) were still in colour and only update to B&W if I leave them showing in the grid or filmstrip for a few seconds.
On this basis, your sync should have updated the database entries for all your selections, even though the thumbnail previews don't all update until you scroll through them. You need to re-examine that export that you did....if you are certain that the sync had finished and some of the exported files did not show the adjustments, then clearly something is going wrong on your system as that is not the expected behaviour.