Pictures corrupted on Mac from photo stream

I take a lot of photos on my iPhone, and they are saved to the iCloud and synced to my iMac into Photos. This has been working fine for years -- I have over 50K photos, and they are all on the iCloud and on my iMac.

Recently, I have been getting over 50% of the photos landing on my iMac Photos corrupted. I can see the thumbnail, but if I expand the photo it's super-pixelated, or I get a grey screen with a triangle with an "!" in the center. When I look at the same photo on my iPhone, it's fine, and when I look on the iCloud via the Mac's web browser, it's fine there too. When I have videos, always just have the grey screen with triangle with an "!" in the center. I am never actually missing a photo or video, but I'm missing the main contents of many of them.

I did a rebuild of my Photos library on my iMac, which took a few days to run, and then all the photos were good on my Mac again. But new photos from my iPhone via iCloud were mostly corrupted on the iMac again.

I've also run disk utilities on my iMac, and it came up clean. I have over a TB of storage remaining so I don't think that's an issue.

This just started happening a few months ago, so I am suspecting some type of Apple software bug in one of the new releases.

I have an iPhone 7 Plus 256GB, running IOS 11.1 (just updated to 11.2 a few minutes ago).

My Mac info is:

iMac (27-inch, Late 2013)

Memory: 24 GB 1600 MHz DDR3

3 TB Fusion Drive

El Capitan 10.11.6