I don’t understand what you are saying. Are you saying that one can or can not lose one’s salvation after sinning?
Don’t you think that’s an important question for a believer (the kind described in John 6:28-29) to have a firm “yes” or “no” reply to?
Or do you think that passage somehow answers the question? If so, would you explain it to me please?
Why the qualifier of 'sinning'?
One does not 'lose' their salvation; they throw it away.