Installing a bank app from an extracted APK fails

I want to keep using my old banking app and want to avoid any updates. So I exported (extracted) the current app with ApkExtractor, but I realized that I cannot reinstall the exported APK file. After some digging it turned out that ApkExtractor exported just the base APK which was insufficient for this banking app. So I went for Amaze which is able to export the split APKs as well. So I exported the entire APK set of the banking app, combined the resulting APKs into one single APK file with Antisplitter. Antisplitter is useful as it signes the resulting APK as well. Now with this APK file I could reinstall my banking app, at least Android says that the installation was successful. But if I start the banking app it throws a security error message. What is still missing? The method I described above works fine for several other split APKs like Facebook, etc., but it doesn't work for my particular banking app. I understand that banking apps are sensitive, but I would like to understand what prevents the reinstallation. Can it be something with the signature? Is so, is there any way I can directly export the original installer APK from Google Play? Unfortunately it's unavailable in APKMirror, APKPure, etc. I know I should use the latest banking app, but I still would like to understand what is going on here. Thanks!