I just managed to solve the problem for me (as far as I can tell with the testing I have done so far). I had the same app with two different ids on my phone, one for development, the other for (beta/performance) testing. They both used the same id resulting in some kind of conflict when accessing/writing data with mmkv. In any case, deleting the preduction build solved the problem. I will simply use two different mmvk storage ids to avoid the problem going forward.

Thanks again @youssdevx for the answer.