Hi Jason. Yup, I've seen this as well. I'm hoping that the regression came in with the interpretor panel, which caused some other confusing side effects as well...
https://trac.torproject.org/6439
Unfortunately this issue doesn't have a good, reliable repro besides 'wait for a week and see if it happens' so I'm not yet sure if this addressed the issue or not.
Cheers! -Damian (arm's author)