Found and resolved the problem.
It seems that the iser interface of the AC 300 has a bug. It defaults to 3 A (which corresponds to the 670 W load I mensioned above. However, if you change to e.g. 5 A, it works properly resulting in the expected 1150 W loading power. From that moment you can change to the maximum of 10 A which results in properly charging ~2300 W.
@Bluetti team: this bug should be fixed with an update for the firmware of the AC300.