7-14 psi turbo boost when engine compression brake engaged

Since the engine is still running, there are exhaust gasses in the manifold. Also with the brake engaged, the exhaust pressure will be high, and therefore the turbo will be spinning.

However, I'm not sure what your max turbo output pressure should be. If your wastegate is set for maybe .5 bar (about 7.35 psi) perhaps your wastegate is not dumping. You might check on the correct blowoff pressure setting.

If the setting is 14 psi or so, then there should be no problem.