Interesting, I hadn’t seen that particular log pattern before…
The Fly.io platform, I believe, is generally always allowed to leave your Machines in the stopped state instead of suspended, so your code should really be ready for both outcomes:
https://fly.io/docs/reference/suspend-resume/#snapshot-behavior-with-suspend
https://fly.io/docs/reference/suspend-resume/#limitations-and-considerations
(I read that as implied by the above, anyway.)
Many people are particularly surprised by the way in which this behavior interacts with deploys. (The above doc notes that deploys result in a “cold start”, but a large fraction of users seem to interpret that as meaning that every Machine will be restarted immediately, as part of the deploy.) This comes up in the forum every few months.
https://community.fly.io/t/machines-still-stopping-instead-of-suspending/27115/7
It doesn’t look like yours was associated with a deploy, but perhaps it was falling more under the “space reclamation” clause mentioned in the doc, 
…
(Some of the underlying physical host machines have been really short on capacity lately.)