Why we have 2 usernames for the same external user (Guest account)

Hi all,

I have shared a sharepoint online site with an external user email address which is (for example) test.abc@org123.com, and the user received the invitation email and has accessed the site few days ago.

But today the user start getting an error that his external user does not exists inside our tenant, something as follow:-

so from "Microsoft 365 admin center" >> "Users" >> "Active users" , i can see that there are 2 entries for the external user email (test.abc@org123.com), as follow:-

1. the First entry has its username with small letters + without the word "onmicrosoft", as follow:-

- test.abc_Org123.com#EXT#@ourtenant.com

2. the Second entry has its username with 2 capital letters (T & A) + the username contain the word "onmicrosoft", as follow:-

- Test.Abc_Org123.com#EXT#@ourtenant.onmicrosoft.com

so why the user is getting an error that his email address does not exists + why we got 2 usernames for the same external user which i shared the site with it (test.abc@org123.com) ? although i shared his email address test.abc@org123.com and he received the invitation email , but when he click on the invitation link he will get the above error..

Regards