"unlikely to be anything wrong with the formula":
As soon as you open a new container of powder, you are letting whatever bugs are in the environment in it. Even the conditions in which they are sealed will not be 100% sterile.
Each time you dip the scoop in the powder it potentially introduces bacteria into it. The only way to kill any bacteria is to mix it with water that is 70 degrees or more.
Mixing with cooled boiled water means the water is safe - but the powder isn't necessarily.
Making feeds in advance means giving any bacteria time to multiply which is why it isn't recommended.
Cartons don't have these problems as the contents are sterile.
The old advice used to be that feeds could be made and stored hours in advance. Whether or not people still choose to do it that way is up to them but there is an increased risk of a stomach upset to a baby which in some cases could be very serious.
I personally have made a feed up approximately half an hour before I will be using it. This is making it up as per the guidelines, cooling it as normal then keeping it stored in the fridge in a jug of chilled water. If a feed was due at 11, say, I might have it ready and stored in chilled water, for 10.30 or 10.40 (no longer than half an hour in advance). This is not the recommended guideline.
This was only once the baby was 5mo as I felt he was a bit hardier then. Prior to this it was timed to the minute, or I used cartons.