I don't know the law in Canada, but the law and constitution in Ukraine allowed it. Conflating the law in your country to Ukraine's is an error.

Interestingly enough, the original US consititution was vague on the point of secession. Thus the US South claimed it was their right to secede from the union, and the north only proved their "right" by winning the civil war.

After the US civil war, US law was changed to make it clear that once a state entered the union, it did not have the right to secede.