Why do some downloaded images lack date/timestamp information in ExifTool?

Maybe the images don't have internal metadata date, and Photos has just displayed the file date that happened to be almost correct when the images were imported? Many social media sites like Facebook, WhatsApp etc strip metadata from the images.

Or maybe the workflow you used to "download" the images stripped internal metadata dates? Drag and drop from Photos might output very unexpected results so use File > Export... instead.

FWIW I usually check the dates with something like:

exiftool -a -G1 -s -fileOrder5 FileName -api QuickTimeUTC=1 -api RequestTags=FileCreateDate -Time:All .

acousticroots74 wrote: … Not all images are in iCloud, and not all match what's in my Photos app,

Are you sure of this? If so, then it is quite worrisome. It could be because you're out of storage. The iPhone should have gigabytes of free storage or there won't be enough room form Photos to be able to manipulate pictures. And when storage gets low at iCloud.com, the same thing happens, and pictures can't be updated.

Another possibility is that some pictures were acquired by the iPhone's Photos in a manor that kept Photos from having full control over the image-- that can happen when pictures are transferred by cable from a PC , for example. Photos can show the picture, but it can't do much else.

You don't say what iPhone you're using or what iOS, or if you are also using a Mac and what OS for that. You also didn't say if you are using "Optimize Storage" for Photos on your phone (or Mac, if you have one.) But I think that you should pay attention to these pictures (if they're important,) and get them off the phone before they disappear. Unless you have them backed up to an external drive, you can'd depend on them doing what you think you are telling Photos to do with them.

You especially need to make sure you have plenty of free space on your phone and at iCloud.com. You need to buy extra space for a month while you clear things up.