Hi, would it be possible to add a confirmation window [Challenge Log objective xxx complete. Would you like to receive rewards? Yes/No ]

If player choose No, that objective's progress will remain at 4/5 or 9/10 and so on and the latest step in progress will be disregarded by the game. You can come back later on a job that can receive full rewards of the challenge and complete that missing objective on that job. This way people won't complete objectives when they don't want to and devs can keep their "we won't let you choose what job receives xp rewards!!!11".

As the name says, a reward is something beneficial that should make you happy and constitute a gain. Unintentionally completing a Challenge Log entry (available once a week only) on a lv50 job when you could've done it later on your low level job feels very disheartening and totally not a reward. More like "Haha, got ya there, you idiot!"