ISO-8601 does not allow replacing the T with a space. You are thinking of RFC 3339 “Date and Time on the Internet: Timestamps”, a profile of ISO-8601 that does allow replacing the T with a space.
> NOTE: ISO 8601 defines date and time separated by "T". Applications using this syntax may choose, for the sake of readability, to specify a full-date and full-time separated by (say) a space character.
From https://tools.ietf.org/html/rfc3339#page-8:
> NOTE: ISO 8601 defines date and time separated by "T". Applications using this syntax may choose, for the sake of readability, to specify a full-date and full-time separated by (say) a space character.
See https://en.wikipedia.org/wiki/ISO_8601#Combined_date_and_tim... for more references.