Here my extensability:

{
"plugin": {
"url": "https://github.com/wrobins/cordova-plugin-msal.git#v3.0.0-alpha.0",
"variables": [
{
"name": "TENANT_ID",
"value": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
},
{
"name": "CLIENT_ID",
"value": "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
},
{
"name": "KEY_HASH",
"value": "S0m3K3yh4shH3re="
}
]
},
"preferences": {
"android": [{
"name": "AndroidXEnabled",
"value": "true"
}]
}
}

I looked into this further, and it seems that the Gradle version on MABS 6.3 is having trouble with one of the dependent libraries for MSAL. If you're able, try building with MABS 7 which should resolve your issue. I'll also update this plugin's documentation.

Let me know if this works for you.