> Apologies- in the event that this patch is satisfactory, the following
> includes `xref-find-references-and-replace`.  I didn’t realize it
> would be a simple replacement.
> [...]
> @@ -1072,9 +1072,11 @@ project-query-replace-regexp
> -   (pcase-let ((`(,from ,to)
> -                (query-replace-read-args "Query replace (regexp)" t t)))
> -     (list from to)))
> +   (let ((read-regexp-defaults-function (lambda ()
> +                                          (find-tag-default-as-regexp))))
> +    (pcase-let ((`(,from ,to)
> +                 (query-replace-read-args "Query replace (regexp)" t t)))
> +      (list from to))))

The problem was in let-binding 'read-regexp-defaults-function' to a lambda.
It should work when using 'find-tag-default-as-regexp' as a symbol: