MobileAds.jar + MobileAds.xml is an outdated method that has long been incompatible with how the current Google Mobile Ads SDK works. Currently, the standard approach in B4X is FirebaseAdMob2 / Google Mobile Ads SDK v20+, as mentioned in that thread. Otherwise, you may encounter incompatibility issues, strange errors, and problems with impressions/metrics.
And if the goal is not just “to be displayed,” but to actually monetize consistently, it's better to go through mediation right away (I ended up going with CAS, and there were fewer surprises).