It has now been revealed by Magny himself that a failed drug test administered by the United States Anti-Doping Agency (USADA) is what lead to the fight withdrawal.
Magny would reveal the news through a post on Instagram:
As many of you know, I had to withdraw from my sceduled bout against Vicente Luque on Saturday, May 18th. I want to apologize to him, as I know how difficult it is to lose an opponent days out from a fight. Throughout my MMA career I have been very transparent. I am not afraid to admit when I am in the wrong. On Saturday, May 11, 2019, I recieved an email from USADA stating that I have failed an "out of competition drug test" due to the banned substance "Di-Hydroxy-LGD-4033". I am fully cooperating with USADA to determine how this substance was found in the sample I provided them on May 5, 2019. I have always been an advocate for the strict drug testing in the UFC, even to the extent of opting for my collected samples to be used for research purposes by USADA. I know without a doubt that I have done everything according to the standards set by USADA. I have faith in USADA that this situation will resolved in a timely manner and that I will be cleared of any wrong doing. To all of my fans and supporters, thank you. I assure you that I have not let you down.
There has been no comment from either the UFC or USADA on the matter, it is also unknown at this time as to how long Magny could be out of action for.