The Church definitely has perfectly sound theological reasons for mandating fasting before communion, but this a merely ecclesiastical law at the service of what is greater; namely, Jesus’ words in John 6:53-54:
Very truly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise them up at the last day.
In addition, because this law is merely ecclesiastical, any sin involved is not due to the actual physical act of failing to keep the one-hour fast before receiving the Eucharist, but due to disobedience to the Church. These kinds of situations depend on interior disposition; if the person’s intention in violating the law was to show disdain for the Church or because of disobedience, then yes, it would be sinful. If this is not the intention, then no, it is not sinful if one is off by a margin of ~5 minutes.
And if one is off by a few minutes, in my opinion they should receive anyway provided that their failure to keep the fast is not because of disobedience. Receiving the Eucharist is far more important than keeping the fast to exactly one hour.