The native mobile builder has been very unreliable for me lately as well. Additionally, it can take over an hour for it to throw an error code for me once I have initiated the build. This is very unsustainable for our development timeline.
We have started building our app packages locally using android studio and xcode. With this process we can have app packages created in under 5 minutes locally. The process for locally building an app is outlined in the documentation here. From there, Android studio can generate a signed APK/AAB with relative ease.
While it is less convenient than having the native mobile builder automatically add the project to app center, it is the only method we have found that makes deploying a native mobile app with Mendix feasible.
This does not directly answer your question but, when you have a client depending on the native mobile app to be delivered, the native mobile builder has not been a reliable enough tool for us to use in our workflow.
Hope this helps!
Indeed, the task of generating native mobile APK/AAB had some instabilities in the last few months, mostly caused by some upgrades in the React Native dependencies.
As Danny commented, I personally almost never build my apps using AppCenter. I Always prefer local generation. I'm trying to automate this process using Python as a tool to generate the APK. Please check my article on Medium:
Using Python as Tool to Build APKs From a Native Mobile Mendix Project
I'd appreciate any feedback!