I don’t think it’s just you, but I’d never heard someone refer to the registration decal as a tag before you.

I asked ChatGPT in the most neutral way I could think what “tag” meant in relation to motor vehicles just to get an aggregate perspective on what the internet at large says and it turns out both are definitely common:

I wonder if it’s geographic or what. Its odd to me that we both had only encountered one usage and it was the opposite