Bertram, Poppy and Cyril all go missing in February of 3633, the year the Plot sort of officially starts. Bertram and Poppy were together when it happened (the two were acquaintances) and so it’s assumed that they went missing for the same reason. Evidence points to kidnapping as they found the two’s pokemon left in the side street the two had been talking in in Lumiose. Cyril goes missing about a day later, while travelling in Lumiose with Anise, but his disappearance is not linked to the other two for a while. 

As for what happened to these three, as mentioned (in the tags I think) on a previous post, there is a danger in this world of people with supernatural abilities being captured and sold on the black market for a variety of horrifying ends. As Bertie, Poppy, and Cyril are all mages (and Bertie and Poppy are also dragonbloods to boot), they were targeted by a group that kidnaps people to sell underground (known as ‘mage traffickers’.) The trio end up on auction and are all bought by the same man, a foreigner (probably Galarian or something) named Harrod Levesque who keeps them as his ‘pets’ in his fancy manor house. He covers his tracks well, and it takes a little over two years for their families to track them down and rescue them (despite one of them being the son of Champion Iris, this stuff runs deep), along with the twins Bertie had in the first year of captivity, Aurel and Ophir. Bertie and his boyfriend, Lunen, raise the twins with help from their families, while Harrod is sent to jail. Ultimately all three of them are obviously traumatized, but they all do their best to move forward and are able to lead fulfilling lives.

This subplot ties into the ‘main plot’ as, while searching for what happened to these three, multiple characters end up discovering the threads that lead to uncovering the existence of the ‘main threat/villain’ of this AU, which kickstarts the lead up to the final battle, etc. (I’m still working on this plotline though.) This plotline also leads to more awareness of the threats mages and supernatural beings face when they are treated as commodities and leads to reforms across many regions, though starting in Unova thanks to Iris.