Something you just said made things fall into place.
A new bespoke TPN bag was ordered from pharmacy.
That MUST surely mean that they were using a stock bag temporarily until a new bag arrived. If they had reconnected the first bag, there would have been no reason to change it (given, of course, that they had no idea or thought that the first bag was contaminated and would not have any idea for at least another week)
Instead, the intention was to replace it with one tailored for the baby. Meaning that the bag used between 10am and 4pm had to have been a stock bag.