Comment 1 for bug 1876906

The /tmp case is a special case. The snapd security sandbox doesn't allow snaps to share/leak files in the system-wide /tmp, so when you select /tmp in the file save dialog, the file is in fact saved to /tmp/snap.chromium/tmp/, which isn't readable by other snaps (and not even by the current user outside of the sandbox, for that matter).

This does mean that you cannot interact with /tmp as you're used to, and is a conscious security/usability trade-off. There's a discussion about this in bug #1790608.

If you choose to save downloaded files to anywhere else in your home directory, or some place under /mnt or /media (provided the removable-media interface is connected), there should be no problem opening them.