Property tax 2018

If you were already registered Belgian residents at the time that the acte de vente was signed (and therefore the property became yours), the tax demand has probably gone astray.
If you were new arrivals in Belgium, so that the home you bought was your first registered address, the tax demand has probably gone to your previous address outside Belgium. This is because, at the time of the purchase. you had no OFFICIAL address in Belgium, because you can on.ly acquire an official address when the registration process has been completed, which can take several months for someone newly arrived in Belgium, and in the meantime anyone who wishes to communicate with you at your official address has no option other than to do so via your old address. In theory, this should get corrected when your registration is complete, in practice it often isn't.
Either way your commune should be your first port of call as they will be able to tell you who deals with your property tax and how to contact them.

I also bought an apartment la2017.
You do not pay anything on the 2017 itself as the quote is calculated by the Notaire and added to its bill when you seal the deal (Act d'achat) but you definitely had to pay the one for 2018. I did receive mine between august/september last year.

You should get in touch with the tax office responsible for that, possibly in a written form as a proof in case of future issues.

The tax is levied on the person registered as the owner on 1 January so you would not get a bill for 2017.
We had a similar situation. It was into the second calendar year of our ownership before we got a bill for the previous year and, 3 or 4 months later, a bill for the current year.
I would ask the notaris first. If the tax office or commune have made an error you don't want to remind them!