Set ETA doesn't change order modified date

We're working on an integration to feed order updates to an external system. We went down the path of implementing webhooks (v1.0 API) and quickly found that there were limitations to the events supported (e.g. change in ETA and change in leg delivery window weren't covered by webhooks).

We changed our approach to polling at a fixed frequency and searching for changed orders and changed legs. This has worked for the most part except for one use case.

When Set ETA is used in the application on an order that is in a Booked status, the updated_at date on the order does not change. If the order is in an In-Transit state and the Set ETA & Location is used, the updated_at date on the order does change. This behavior looks like a bug.

Is there a workaround? How can I identify orders where the ETA has been recently set (assuming the order hasn't been changed otherwise)?