The API for Instant.toString() says this:
A string representation of this instant using ISO-8601 representation.
The format used is the same as DateTimeFormatter.ISO_INSTANT.
The latter in turn says this:
The ISO instant formatter that formats or parses an instant in UTC, such as '2011-12-03T10:15:30Z'.
You probably missed that Z which is the same as +00:00, UTC and GMT.