Fixed it! I went ahead and re-created the test case that fails to generate a correct toolpath in v1.42a in previous versions and found that rest machining worked fine there. I was then able to deduce that there was a code change from v1.40a to v1.41a that was missing an additional line of code that was needed, resulting in rest machining being broken in certain situations.
I'm working on some new canvas editing functionality at the moment and once they've been knocked out v1.43a will be rolled out, including the fix for the rest-machining bug.
Thanks again for the bug report!
Charlie