Import watch-only address to 0.18 core wallet, addresses missing
You're using on incorrect bitcoin-cli method to list your addresses.

The command you're looking for is:

bitcoin-cli listreceivedbyaddress 0 true


The 0 parameter means you're requesting information on any address and the true flag means you want empty addresses included.

The original command you were attempting, listaddressgroupings, is described here.