A turbo will always generate slightly more boost in each gear with 1st gear generating the least and 5th/6th generating the most, no matter how tight the mapped boost control.
I would expect a car mapped to 1.5bar to only generate about 1.35bar in 2nd.
A car mapped on the dyno to 1.3bar in 4th would generate about 1.35bar out on the road in 5th. Maybe more depending on the dyno load setting.
The boost control for the solenoid/wastegate has a max and min % setting (dependant on ECU model/code). If the gap between max and min is too small or when running the minimum mapped % of wastegate control (some models only have 1 map for wastegate control) causes the boost to overshoot target the safety features within the ECU will cause the boost control will be switched off.
As soon as boost control is switched off boost will drop to around 0.7bar. But as soon as this happens, because it has dropped below target, the boost control will switch back on and the closed loop control nature means boost will surge up and down.
You should find that if you slightly back off the throttle that boost stabilises.
This is due to the throttle position being mapped to a region that has different boost control % that enables stable boost.
As mentioned above its quite common and why when road mapped it's always best to squeeze in a 5th/6th gear pull from around 2500rpm and also a part throttle 5th/6th gear pull to check boost is under control in all scenarios.
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