This is such an oversight by HubSpot. There are people crying out for this functionality. Even some people offering up a relatively simple workaround using custom fields with date stamps and a calculation field that recalculates your own actual open time. But this doesn’t work for more than one “pause”.
The way we use tickets in our business is for spoilage. these should take a maximum of 3 days internally. but we have many tickets that have been open for 60+ days due to customers taking forever to come back to us. You can imagine how messy this makes reporting and how massively it skews average open times.
HubSpot, please start looking at some of these feature requests based on their necessity and merit rather than just by how many upvotes they have. This has been open for almost 4 years!