Hey @Sergei,
If the user is signed in, their user id and user email will override the registrant_id and user_email.
One way to know if the registrant is the participant, is if you invite them with the same email that they are signed in with when they join the meeting.
Then you can call Get Meeting Participants Reports and compare the email with the registrants email returned from List Meeting Registrants.
Unfortunately it is not possible to add some custom unique id that will not change during the meeting. I see the pain point there, and will pass this feedback to our team.
Thanks,
Tommy