Political preferences of 98% voters breached, €65,000 GDPR fine

> The leaked personal information included telephone numbers, dates of birth and, voting intentions and party leanings of over 330,000 individuals affected.

Any clue where this data originates? It mentions a client, but what sort of client would have that data? Do Maltese voters officially register this data with anyone (sounds like a serious breach of voting privacy)? If it were inferred from normal web tracking, it wouldn't be limited to Malta..