I heard that when you launch Bedrock Edition from the Minecraft Launcher, the ID (Gamertags) that is displayed while loading until the sign-in is completed should originally be displayed as "Player".

This is the yellow part of the screenshot.

In my case, for the past few months, I've been seeing an unfamiliar ID instead of "Player" when loading.

It's always the same ID, but it's not mine. Once the sign-in is complete, the ID will be automatically displayed correctly, so there is no problem with the sign-in itself.