That does seem strange
is 2400km 1500miles?

i had a strange one when they fitted the new pump and tank
they said it took 25 litres to fill and that's strange as I only had the 15 litres well so I thought
snd they said the same size tank was fitted that came off really didn't make sense

when I had my first warning of refill in 1500 miles I put back in 8 litres
so def had the smaller tank think the tank was changed around 4000 miles and sold the car at 8,000 miles
so never refilled tank or had chance to see when the next warning came up

i don't think my mmi unit says when to refill it just comes with a warning to refill in 1500 miles
up until that happens you don't know when it will pop up

sounds to me they either just refill your tank with the old ad blue that came from your old tank
bad if they did or they didn't top it up fully

M