Is this a plug and play receiver or one built into the truck? If it's built in, is the truck new, used or certified pre-owned?
I believe most, if not all manufactures include at least 6 months of Sirius XM on new cars. I know last year when I was looking at vehicles, Ford was offering 6 months free on certified pre-owned. I would assume other manufactures do the same. If it was used but not certified, the radio didn't receive the deactivation signal yet from the previous owner. Both my wife and I bought non certified used vehicles last year, I went a little under a month before I lost my free Sirius XM, my wife went about two months.
Same goes for plug and play receivers. If your receiver didn't receive the deactivation signal yet, you'll be able to use it until it does. I have an old Delphi SkyFi 2 that I deactivated about 8 years ago and used to use sparingly and it still worked as of maybe a year or two ago.
So yes, this is normal, but don't expect it to last forever if you have the receiver powered on a lot.