Unknown participant "t8" can appear on a Teams CVI call when a "Teams Consumer" participant joins a Teams meeting.

We've already seen a reduction in the Teams Consumer recorder bots being added to meetings, and therefore the "t8" unexpected user is now very rarely seen on screen. A fix will be applied to the service within the next few weeks.

Further explanation:
The unknown teams participant (in some layouts, shown as the "t8" participant avatar) can appear to the Video Conference (CVI) participant, when a "Teams Consumer" user joins the meeting.
The "t8" participant represents the Teams Consumer meeting recording bot, and should not be visible to the Video Conference (CVI) participant.
There is no impact on meetings when only Teams Enterprise / Commercial / Business users are present.

The issue can be seen on meetings where a Teams recorder bot is present.
The unknown participant represents a new version of the Teams recording bot.
We are working to block visibility of this new bot in the layout ASAP.