I don't do it that way-- I pop the SD card into an SD card reader. The SD card acts like a disk drive, and i just drag the files to a Finder folder. But I tried using Image Capture (a pretty faithful app!) with my phone, and I could select the download site as usual. I'm in 15.6. Can you see if it works with a card reader-- that would eliminate the camera as a problem.
Do you use iCloud Drive? Do you have Documents in iCloud Drive? I do, but it worked anyway, so just checking…
Did you try other folders? What happens? Nothing, or it goes to the wrong place and pretends that it worked? or what?
To narrow things down, try these steps:
- Restart the computer (of course)
- Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
- Make a new user and see if the same problem recurs with it. This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder.
The idea is to find out if the problem is with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.
Let us know what happens…