All I can think of is that it's over boosting past 1 bar, then there is a drop of value which basically protects the turbos.
E.g. my HKS EVC-S is set to boost to 1.1 and 1.2 bar, even tho it's set to that sometimes it overshoots. There will be an overboost value of either 1.2 or 1.3 set depending on my mode, and if it hits it, it drops the boost all the way back
It could also have the fuel cut defender in place, if boosting over 1 bar for more then 3 seconds it will drop back. Usually there is a little black box or HKS FCD fit where the ecu is. If it doesn't have one then it could be the issue
Try turning the boost controller off entirely and see what happens. If it still cuts the boost at 1 bar then i would suggest it doesnt have a fuel cut defender.
Also - in the colder dense wether it will / can overboost. A boost controller can only raise boost, not lower it