That looks like an opaque user ID? Did you request the user ID from the user? If you’re testing with an account where you are the broadcaster and you access your own channel page, you will be given automatically your actual user ID in the JWT payload as Barry mentioned. In other words, you don’t have to request/share your own user ID, but you do for everyone else.
Another cross check is during the times you see an actual user ID, does the JWT payload show a role field with the value “broadcaster”, indicating the current user data in the payload belongs to the channel owner (i.e. - the broadcaster).
One point of interest. Are you seeing that user ID in the JWT payload user_id field or in the opaque_user_id field? I’ve only seen user IDs that are both letters and numbers in the opaque_user_id field. All the user_id field values I’ve seen are always numeric (so far), except for the “U” that prefixes them when used as a user ID instead of a channel ID.