I confirm the problem. There is a missing step in Discoverer's report which is fairly obvious but should still be stated to avoid confusion:
- Highlight some code in the Arduino Web Editor.
- Right click on the highlighted code.
- Click "Cut". The selected code is deleted.
- Compile the code, the cut line is still compiled.
The same does not occur if you use the Ctrl + X keyboard shortcut.
Thanks for reporting this Discoverer!