Removing a custom field with duplicate name breaks JQL search, perhaps a stale cache?

Setup
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We had one custom field named Progress of type Unlimited Text. It was restricted to one project and one issue type.

Then we added another field named Progress of type Unlimited Text and only restricted it to the issue type, so making it used by all projects.

We then deleted the second field, so now there was only the one original field named Progress.

We did not reindex at any time, at least not using the Admin, Index screen.

Problem
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A JQL query with

"project = MYPROJ and issuetype = MyIssueType and Progress is not empty"

that worked before the changes now produced the error:

"Field 'Progress' is not searchable, it is only sortable."

However the following JQL query worked without errors:

"project = MYPROJ and issuetype = MyIssueType and cf[10200] is not empty"

10200 is the id of the original Progress field.

Which suggests to me that something stale can appear in the place where JQL looks up the custom field based on the name of the field.