Having the alert does not seem to make any difference to sales or winning the buy box. That’s not to say it won’t in the future.

However with all those we have changed (for non-white backgrounds) the alert has disappeared.

It is difficult to say whether changing it has any effect, as you never know what other factors are in play. I haven’t had any listings removed because of non white backgrrounds yet, although I am gradually changing them as some of these images are poor quality as well.
Like you I find that the images are not always uploaded by Amazon and then have to complain via Seller Central and be willing to show proof of validity, even then a few refuse to update and I have had to take this further with them. Annoyingly foreign sellers sometimes seem capable of overwriting images & descriptions with poor quality from other Amazon sites, reversing improvements I have made.
By the way it seems to be the main image affected by this ruling, so you can add lifestyle, non white background images as supplementary ones without issue.

In my person experience (Worked for several amazon account ranging from top 100 sellers to top 10000 seller)

It seems Amazon crack down on these white background images and other policies the higher rated seller you are.
Until you reach something like 1000 ratings p/m Amz should leave you alone

In my experience, its the higher the product ranking, the more likely it is to get suppressed.

For example, I had a tie in 3 colours, which due to the nature of the design was very difficult to photograph in the standard ‘curled up’ way, so I had to crop the image.

All was well until the listing started selling quite well, or one of the colours did. Amazon cracked down on this individual one due to it being cropped, but the other colours photographed in the same way were fine.

It’s been the same for me with a few ‘non white’ background pics too, as soon as they start to sell, they get suppressed

I’ve got about 130 listings suppressed due to “non white background”.
Problem is that although the images that I have uploaded do have pure white background #ffffff the Amazon catalogue is still clinging on to another seller on the listings images.
Only way to clear them is to contact SS with each ASIN confirming that our images are compliant.

Even then we sometimes get a message back saying our images aren’t pure white background and that they have a blur around them - when in fact that’s just the compression of a jpg showing artefacts on the edges.

Listings with Quality Alerts are definitely performing worse than the ones without them. They have a worse placement in the search results. And this applies to all product categories.

According to my own experience, as soon as I cleared all the alerts, the listings which normally weren’t getting any sales started selling.

You can easily fix the non-white backgrounds to meet the Amazon standards by using various free online tools, such as Bonanza Background Burner or Magic Eraser for iOS.

Nearly our entire inventory of SKUs has got quality alerts about the images, white background or not.
We can’t find any evidence to say it affects sales performance though leaving them on, but I imagine one day in the future it will.

Our three best sellers are all lifestyle images and FBA

Had a few,I just put a white border round them & the alert disapears.

If the photo rules were applied rigidly in the Books category then at least 50% of the catalogue would disappear. I think it would cause a major problem for some of the mega sellers