If a lead keeps getting assigned to a user other than the user that created the lead, this can troubleshooted by checking the company settings.
Go to the Sales Company > Company Settings
Find the "set default lead owner?" question to see if there is a rep listed. If there is one listed, that is who all leads will automatically be assigned to after a lead is created.
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This can be removed by removing the listed name and replacing it with "Select Option"
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This will then allow created leads to remain under the users that created them.