Even if a stock bag was used at 12pm, is it really that far fetched to think Lucy could have tampered with it before she left for her shift? (Given we know someone definitely poisoned that baby so would be that way inclined).
How are the bags stored in the fridge and what is stock rotation like as I assume they have expiry dates? She could have taken a guess they may use a stock bag with baby F as she would know he would be unwell from the original bag and doctors would be doing different treatments, so tampered with the one in the fridge that would most likely to be used next. If they didn’t use it for baby F then it would have been used on another baby instead and maybe she would have just had another victim.
In the context of knowing someone on the ward was deliberately harming babies I don’t think it sounds that far fetched