Caveat: I am not a legal expert. In my opinion I have a somewhat more informed view having taken courses on the Law of Armed Conflict (which included looking at the Hague and Geneva Conventions, etc and how they relate to conflicts).
Absolutely no where is it stated that military members have to wear a nations flag anywhere, or at any time. I'm paraphrasing, but the requirement relates to "does this person qualify for PW status under the convention" - a military member must be able to be distinguished as a member of a military force of a nation (wearing a uniform that set it apart from other actors). Wearing CADPAT would meet this requirement. Military members are even allowed to not wear a uniform, but run the risk of being considered a spy and conducting espionage (which invalidates their right to PW status).
Without going too far down the rabbit hole - it is true that Canada has not ratified all the protocols and amendments that would fall into your argument. I'd suggest cracking open the books to fully grasp the absolute claim you are attempting to make. There is a reason why we have Legal experts who deal with these exact questions - they are (usually) not black and white.