This problem appears limited to Hazel on Mojave (running 10.14.2 beta, but also a problem with 10.14.1) and the ScanSnap Home software. The rule has the condition "Kind is Image" and the file is a jpeg created by the ScanSnap scanner. The rule fails to match unless I do something with the file first, like opening it with Preview. If I just leave the file in the folder it never gets processed by Hazel correctly (matching "Kind is Image") even if I force rescanning. Jpegs from other sources placed in the folder get seen by Hazel just fine. Note that if I use the Spotlight search in Finder for Kind:Image it sees the file.
I'm seeing other rules fail to match correctly, at random with this software/scanner combination as well, but the Image problem is consistent.