There's no way you aren't trolling lol.
Default units in both Design and Manufacturing are set to inches:


I'm in inch mode and it's set as the default:

Surely every number I input should be assumed to be in inches unless I specify otherwise?
I brought this up regarding the tool library a few years ago and it was either agreed it should be changed or it was already a known issue I can't remember.
Why would it make sense that Fusion would correctly assume the first value is what the user set Fusion to but then just change the second to mm?
How is this not ripe to make users make accidents? I'm always double checking stock size and going "...why is my stock .37189834" thick? Oh..... It wasn't enough that I told Fusion once what units I want...
In the first part of your math, you've specified the units (-.33 in)
Fusion specified the units. Because I'm in inch mode. I never typed "in". I would have just typed "-.33"
What do you get when you have -.33 -.125
.325, treats second as mm.
There's no way you aren't trolling?
Autodesk's most dedicated bug reporter: https://forums.autodesk.com/t5/user/viewprofilepage/user-id/15188348