This is a weird request by IRCC. How can an employer promise to have anyone employed for at least one year. It depends on what the employee does, legal status, performance during the year and a lot of such factors(including company's existence!). Employer might not always be ready to mention on the letter saying I will employ you for at least one year post PR. Also, what if the employee doesn't like working there anymore and what if the employee wants to leave? The employer may put us on the hook saying that I will give you the letter but you have to work for one year after the PR or something of that sort. It is not a good request by IRCC if you ask me.
I am in the same boat as the OP(haven't received a rejection yet though!), and I have provided a LOE stating that my job is full-time, permanent and it is valid for as long as I am eligible to stay/work in Canada legally. In addition I have provided employment letter and all other good stuff which also mention that I am full-time, permanent. I don't think it is fair to ask my employer to promise IRCC to keep my job for one year after the PR approval.
