Has iStock implemented a 'stacked images' policy similar to the one used by Getty, where only one image from a series is actually shown in search results?  Has there been any announcement about this?

Today I looked at my portfolio sorted by 'newest' and was rather surprised to find that more than half of files uploaded in recent months are not visible.  More specifically all the similars are missing.  If I'd uploaded a small series of four or five, only one of those is actually shown in my portfolio.  When I click on that image I can see all the others in that series shown in the similars carousel.  But they don't appear in my portfolio and I cannot find them if I search.

This appears to be similar to Getty's 'stacked images' system where only one image from a series is shown in the search results, and the others are only visible as similars.

Has anyone else noticed this?  If implemented across the board, this could eliminate millions of images from the search results and have significant implications for sales.

I noticed it since July.  They are all there, but not on first page when you search by NEW (in classic view).  Uploading complete series together is probably not a very good idea any more.

If you want your photos be seen DON'T upload similars in the same batch.I don't know if it is the inspectors or automatic software but it groups similar images. If you have this problem with already uploaded images....deactivate and upload again scattered in different batches.

I have seen whole batches swallowed by this new system where only a few are shown of dozens

That's right, you reminded me it was introduced after a series of complaints about so many near-identical photos hogging the top of the New, and occasionally the best match, sort orders (sometimes in some browsers, New and best match are identical).
Rather than stop the near-identical uploads, this is what they chose to do.
I'm not even fully convinced any more that they even have an Evil Plan.

Since the implementation of this 'stacked similars' policy back in July my sales at iStock have been gradually falling month after month.  I imagine as they gradually remove more and more similars, overall sales will just get worse and worse.  I have a sizeable number of Christmas seasonal images that in the past have produced good performance around this time of year, but none of that usual burst of activity is happening.  It seems that whatever new things they try to do just results in fewer sales.

Looking back to those big sudden changes in Sept 2012, it seems that every move since then has been a disaster for suppliers, and a disaster for iStock too, given its falling financial results.  But still new changes are implemented, and still the sales fall...