Token Copies of Watcher for Tomorrow do not behave correctly when leaving the battlefield

Watcher for Tomorrow has Hideaway, exiling a card when it enters.
It also has a triggered ability that reads "When Watcher for Tomorrow leaves the battlefield, put the exiled card into its owner's hand".

I've found the following interaction:

Have Inalla, Archmage Ritualist in the command zone.
Play Watcher for Tomorrow and pay the 1 for Inalla's triggered ability.
Both the original Watcher and its token copy come into play and each triggers Hideaway, exiling cards.
At end of turn, the token is exiled per Inalla's ability, but the exiled cards remain stranded in exile and are not added to the player's hand.

Start by locating the Java implementations for Watcher for Tomorrow and Inalla, Archmage Ritualist, then reproduce the described token-copy interaction. Trace how Hideaway's exiled card is associated with each permanent and what happens when the token leaves; done means both exiled cards reach their owners' hands as specified.

Written by the indexing model from the issue text.