Yeah so Ace was captured by the Germa 66 cult.
The vibsibs are taken out of the cult young thanks to Sora, but they live on the run their whole life, always on the outskirts of society, never in a single place for long. At first with their mom, after her death, with Zeff, and as adults they travel mostly on their own, meeting and contacting periodically, careful not to get caught by their father or accidentally take his followers to their siblings.
Ace and Ichiji meet because they're both vagabonds and happen to be following the same route. After several accidental meetings in different places, they grow closer and, after Ichiji finally reveals his name and backstory, they start traveling, with Ace keeping quiet about his new companion (boyfriend) to his own friends and family just to be on the safe side.
Ofc, they end up getting found and captured by Germa, who keep Ace as a prisoner so Ichiji will stay obedient and cooperative.
Hopefully, their friends and family will find and rescue them before it's too late.