I expect a straight path down the center of the cut. A 30mm tool does not generate any g-code.
However, a 29.999999999mm tool generates the expected path shown here:

Any idea what am I overlooking?
[edit]
Very similar to the error in https://forum.freecadweb.org/viewtopic.php?f=15&t=25278, I get the following g code when the tool width matches the pocket width.
(Pocket)
G0 Z25.000000
G0
G0 Z0.000000
G0 Z25.000000
OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.12223 (Git)
Build type: Release
Branch: master
Hash: 917e61a3be12f3316d571ebf6fd6e3c537691c14
Python version: 2.7.8
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.1.0
Locale: English/UnitedStates (en_US)