Boolean difference acting like union (checked normals, doubles, and manifold) I'm trying to perform a boolean difference to cut a piece out of this treasure chest lid to change to a pop-in hinge:

The model is an STL from Thingiverse. I created a shape that I want to cut out of the lid in Fusion360, exported it as an STL, and imported it into Blender:

I lined them up in Blender and then added a boolean difference modifier to the treasure chest lid, setting the cutting shape as the object to subtract.

Instead of cutting a piece out of the lid, it looks like it performed a union instead:

The original cutting shape is hidden, so what is visible here is the result of the operation. It looks like it unioned the two shapes.
I checked all the common reasons for problems:

Normals - Normals look fine when I show them. To be safe I opened each object in edit mode and did Mesh &gt; Normals &gt; Recalculate Outside. No change.
Duplicates - I attempted to eliminate duplicate points using Mesh &gt; Clean Up &gt; Merge By Distance for each object. No change.
Non-Manifold Geometry - When I go to Select &gt; Select All by Trait &gt; Non Manifold, nothing is selected for either object.

What else could be causing my boolean difference to fail? What can I do about it?
Here's a Dropbox link to the Blender file.
Your treasure chest 113Mb blend file was to large for my machine to handle you may want to &quot;clean up&quot; the STL before you upload files. Do a &quot;Limited Dissolve&quot;. http://youtu.be/-OynHqYd_1I
Blender stackexchange has it's own file attachment system to help prevent files being &quot;lost / moved&quot; http://blend-exchange.giantcowfilms.com
Also are you going to 3D print this out? if so there's certain offsets when doing a Bool cut you need to take into account. Like making the cutting object about 0.2mm larger.
Are you using Blender 2.91? The &quot;Bool Tool&quot; using the Exact option may also help.
Here's an animated gif of the process using a box and your lid cutter.