Your suspicion is unfounded. The first past the post electoral system often results in such anomalies. What counts is the vote in each individual constituency, not your total vote across the country. It’s perfectly possible for a party to come second in every constituency, with many millions of votes, but thus fail to win a single parliamentary seat.
The USA also has “first past the post” and no such “anomalies” happen(ed) until fraud occurred.
Why defend Labour’s pyrrhic pseudovictory at all?