Ah thanks! Do you perhaps Google Play Protect installed? This application can cause problems with sideloaded APKs. To disable:
Open "Play Store" > Menu button > "Play Protect" > Disable "Scan device for security threats".
I you haven't got that application installed or it still doesn't work, could you perhaps email me at email@example.com? I might be able to send you an APK specifically for arm64.