Normally I'd say that's just boost tapering off like it should, but that's a bit of a big taper... You could check for a boost leak, but also see what your tuner did to the boost and how he has it taper off.
I'm running 19psi peak, tapering to 14ish at redline for example.
On my previous car I had a road tune done and to heighten the feeling of more power the tuner spiked boost. So it felt great and hit boost high and quick, but then boost tapered severely because he had to pull timing after the spike. So it was a lousy tune but initial impressions were great.
18 to 10 is pretty severe though. What kind of tune does the car have?