The thing is, you have to update them with big life changes, one of which would certainly be getting married or getting a common law partner.We didn't lie on my application for PR. I was only 17 years old when my mom applied me for sponsorship. Yes, it took 10 years to get approval. Life happened, I met someone, got a child of our own etc. As per CIC's website, you can be considered a dependent child if:
'IF' we declared that I am in a common-law relationship, that would make me ineligible as a dependent child.
- they’re under 22 years old, and
- they don’t have a spouse or common-law partner.
From what I can understand, you genuinely weren't in a common law relationship (so that part wasn't a lie), but at the same time, the reason you weren't in one was so that you could still be your mother's dependent and immigrate into Canada. Because you knew entering a common law relationship or marrying (and declaring it) would be disqualifying.
So now you've landed and become a PR and now you want to sponsor your partner of 6+ years who wasn't your spouse or common law partner up until now for the sole purpose of you immigrating into Canada and later sponsoring him. I imagine that's something that caught the eye of the officers dealing with your file.
I think you'd benefit from getting a good immigration lawyer.