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At the time I applied for my permanent residency, my girlfriend and I were living in the same household, but we were not in a committed relationship. We shared the accommodation but kept our lives separate—splitting bills, having no joint finances, and not presenting ourselves publicly as a couple. As time passed, our relationship developed and we are now planning to get married. I applied for PR as single, as we did not consider ourselves to be in a common-law relationship at that time. Now that I intend to sponsor her as my spouse, will this cause any issues with IRCC
Not a meaningful discussion unless you are specific and clear about when you began living together, when you applied, when you became a PR, and when your relationship started (as a proper relationship).

Saying you were 'not presenting yourselves publicly as a couple' makes it sound like you were hiding it for a while. Be clear, please.

And yes - it is potentially serious.