
Tags aren't supported natively in the Power Automate Teams connector typing @tagname in the message body just posts as plain text and won't actually ping anyone.
To make it work properly you need to use the Graph API via an HTTP action. The flow needs to get the tag ID from Graph, then post the message with the correct mention format in the body JSON. It's a few extra steps but it does work.
Good step-by-step guide here: https://www.sharepointeurope.com/mention-a-tag-in-microsoft-teams-with-power-automate/