According to a story from Dutch outlet Nieuwsuur, a few prominent kickboxers allegedly failed drug tests administered by the Netherlands anti-doping agency in 2017 and 2018. They all participated in GLORY kickboxing events in those years, and are facing up to four-year suspensions.
The two fighters with the biggest names would be Badr Hari and Hesdy Gerges, who fought each other at GLORY 45 in March 2018. The other two are Brazilian Ariel Machado and Croatian Mladen Brestovac.
While the linked article is in Dutch, here is an explanation of what it entails in English:
Badr Hari, Hesdy Gerges, Mladen Brestovac and Ariel Machado have been flagged for potential post-fight drug testing failures, per NPO 1.