There is no visual difference in the UI between two cases: a) Save to the VM b) Save to the VM and to the disk So when this internal flag 'also save to the disk' is off by some reason (devtools were reloaded) you will get the old version of the file after reload.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.