You are using Issue property Issue updated date for this calculation. It works for Issue level calculations only, as you already identified.
You can define a set of issues where you are using this formula. The best way to address issue set is by using the function Descendants. However, in this case, you would like to apply some filters to get the correct set of issues as the report filters will not work there by default.
Here is an example report in our demo account Issue days overview by current status you can use a measure Average days in current status as an inspiration for your calculation.
The report works with measure Issue status updated date. You can substitute it with measure Issues updated date. The formula filters only open issues as we assume last update information is important for open issues in most cases.
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