I think they didn't check the XML files after adding the new Addons, because the 49X is not the only truck with that problem at least I see that this happen to with the P12, Ford CT9000. I didn't check all the trucks but for sure there are more trucks with the same issue.
@mastercap I tweaked the 49x, my version has power on all wheels, active suspension instead of liftaxle und more, so I can´t really answer that question...
@treneroefc On some trucks, like the P16 if I remember correctly the addon was added in xml as "not compatible"... So it was done on purpose, at least for now.