That's not tracking to you specifically, that's geo location so they're just trying to identify the general area for purposes of showing ads.

With IPv6 chances are your ISP has a single large address block, and then they split out parts of that address space to users in ontario and quebec. Unless the ISP publishes details of where they've allocated those blocks, any external site is just guessing.

With legacy IP chances are they have multiple small fragmented blocks, some of which are routed to ontario and some of which are routed to quebec so they're a bit easier to keep track of unless the ISP decides to move them.

Also when you address changes, the legacy address space is probably quite close to full so your previous address is released and probably taken by another customer fairly quickly. The IPv6 space will be much larger, and so recycling occurs far more slowly.

Either way youtube is just guessing, it only knows that the address space belongs to a canadian isp and it has to use the behaviour of whoever used those addresses last. If the previous user set their browser language to french and searched for french sites, youtube will remember that and is more likely to serve you french ads.