In app upgrades to Blocs plus work a bit different.
Your second license number is used in conjunction with your original one. But your plus one is not displayed in prefs, only your original.
If you see the word plus on the Home / Splash Screen, it’s Blocs plus.
When or if you move to another Mac in the future, to activate Blocs you will first input your original Blocs 5 license number. After that you would follow this guide and input your additional Plus license number.
I hope that helps.
P.s. The steps above (in the link) won’t be available if you are running Blocs Plus.