Whoa! For a minute I was trying to figure out what all that weird stuff on the right side of the engine bay was... pretty weird to see the steering shaft and master cylinder sitting over there.
It does sound as though you have a safety override situation going on at your elevated boost level. A stock US-spec D24T is limited to about 10psi, I think the D24TIC is closer to 12 due to a different wastegate setting. The safety dump valve on a D24T opens at about 12psi, so dumping at about 15 would make sense on a D24T where 12 is the stock pressure.
However, since you say you are actually seeing vacuum indicated on your gauge, I wonder if your situation is not an air dump but in fact some kind of wastegate override or fuel interrupt? Are you sure that the gauge is really measuring vacuum, or could it just be the needle bouncing down into the vacuum range from rapid movement?
There are some diagnostic tests you could do. The one anders suggested would work but needs a little modification if I am remembering correctly. On a D24T, unless the TIC setup is different, the wastegate mechanism is all internal so there is no external linkage you can secure in one place for testing. However, you could achieve the same effect by removing the pressure signal hose that runs from the compressor housing to the wastegate servo. Just slip the hose off on the compressor side, and plug the opening, then with a warm engine GENTLY go for a test drive. As you get up near 15 psi, watch and see what happens. Either your issue will be gone or will still be present, which will mean, respectively, that the symptom is being caused by a wastegate control override at high boost, or by a fuel interrupt or an air dump in a location you haven't found yet.
Note though, I would be careful running with the wastegate disabled -- you could get a boost spike way higher than 20psi under certain load conditions. If you try that test, then be gentle with the accelerator so that you build boost gradually and don't end up with more than you want. Don't want to pop that nice new headgasket you recently installed. ;-)
If your issue is still present with the WG disabled, then you can re-enable the WG and do some more searching for where the safety dump valve could be. In the back of the intake manifold? In the EGR plumbing somewhere? If it is not to be found, then perhaps on the TIC motors this is accomplished by cutting fuel. To test for that, you could disconnect the 12V+ supply to the IP fuel cutoff solenoid and attach a jumper from battery positive, then go for a gentle drive and gently see if you can gently get the boost up above 15 lbs.
