So found out some more info on this.
Probably related to the fact I have tested successfully a StoreKit Rendered ad with the same advertised app id.
When I call endImpression for the first time I get the following logs from the appstore:
error 09:23:02.243885+0300 appstored Existing impression with higher fidelity found. Not adding params
error 09:23:02.244108+0300 appstored Failed to finalize impression for app bundle ID:xxx.xxx.xxx, adam ID:0
And when I try to call startImpression with a new ad I get the "Cannot start an impression since an open impression already exists"
Seems like an Apple bug.
Topic:
App & System Services
SubTopic:
StoreKit
Tags: