Creatures cannot voluntarily enter an Iron Flask
Dale's probably right; it's very dubious that any creature could voluntarily enter an Iron Flask of its own accord. Ordering the recently-ex-occupant not to resist your attempt to put it back won't work either. This order isn't 'likely to result in [the creature's] death', so the Iron Flask rules don't forbid it. But the rules around saving throws have been clarified by the designers to not allow a creature to voluntarily fail any saving throw.
If they could that would be broken
It's broken because it's completely unrestricted. It's like wishing for more wishes. You can order the creature around for an hour - and its attitude towards you is fixed at 'friendly' so it won't even resent you. At least, not until that part of the enchantment wears off.
Which it won't, because all you'd need to do is order it back in the flask.
There's no cooldown saying, for example, 'a particular creature cannot be forced into any particular Iron Flask more than once, ever', or 'for at least 24 hours after its release', or any such restriction whatsoever. If a creature could voluntarily enter the flask, then once a creature is inside one, it becomes a permanent thrall.
There's also no restriction that a creature has to be in the bottle for a minimum length of time before the compulsion to obey takes hold. It serves you for an hour, then an action puts it back in, then another action lets it back out again whereupon it serves you for another hour.