Does roaming add-on get used before the basic 30GB runs out?

I'm roaming ATM, getting close to my 30GB limit which will renew in a week so bought a data add-on to stop worrying about it. But it seems the add-on data is being used rather than what's left of my basic 30GB. Does add-on always get used first?

yes

That's a bit of a rip-off but thank you.