Some more digging on this.
Seems like CRA might make a determination of whether you're really a self-employed independent contractor or, in actual fact, in an employee-employer situation. If CRA decides that the nature of the relationship is actually that of an employee-employer, then according to CRA...
"Employers are responsible for deducting Canada Pension Plan (CPP) contributions, EI premiums, and income tax from remuneration or other amounts they pay to their employees. Employers must remit these deductions along with their share of CPP contributions and EI premiums, to the Canada Revenue Agency (CRA).
An employer who fails to deduct the required CPP contributions or EI premiums has to pay both the employer's share and the employee's share of any contributions and premiums owing, plus penalties and interest. "
https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/rc4110.html
So, if an employer "allows" you to work a contract as a sole proprietor, but the nature of the relationship is really more like that of an employer-employee, then the employer seems to be taking on various risks with CRA.
Seems like your risk is being labeled a PSB, but as best as I can determine, this might not pose a problem for you if all of your income is treated as salary, you are contributing to CPP, (not sure about EI), and you're not trying to make use of various small business tax deductions, tax rates, etc.
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