New Xbox showing older gamertag?

It depends on if the game is already capable of supporting the "new" Gamertag format.

Xbox changed the Gamertag system a few months ago, to allow multiple people to have the "same" Gamertag, similar to names on Discord or Battle.net, by adding a number to it.

Newer games, that already support the new Gamertag format, will only show your new name, without the #number to your friends.

Older games, that do not support the new Gamertag format, will show your old Gamertag for a while.

This is not a glitch.

Xbox is introducing the new Gamertag system, but the transition is not complete yet. During this transition phase you will see both your old and new Gamertag on your console.

After Microsoft has completed the transition to the new Gamertag system, it might be that older games still show your old gamertag instead of the new one.

It depends on the publisher of the game if they issue a patch to make their game compatible with the new Gamertag format.

Upcoming games or games that have been patched for the new Gamertag system should show the number less prominently.

Older games that have no concept of the new Gamertag system will always show the number "normally".

Sorry about the inconvenience this has caused.

You can not remove a classic Gamertag since some older games rely on the old Gamertag system. Newer games and Xbox One system and Windows 10 will show your new Gamertag.

Xbox changed to a new Gamertag system several months ago.

The new system allows multiple people to have the "same" Gamertag with a number attached to it (like Discord or Battle.net).

However, this new Gamertag system is not yet compatible with older games. Those might still show your old Gamertag, until the games get updated.