Is it possible to reroll a winner if the key was found to be limited by region?

I know the sg moderators tend not to reroll as there is a reasonable chance the same thing may happen to the next winner as I have encountered this issue before. They will probably advise you to remake the giveaway with region restrictions.

If it turns out to be region restricted you need the consent of the winner to delete the giveaway and create a new giveaway with the proper region restriction.

If you explain in the giveaway description most of the times a winner will agree to it, but that's not a guarantee. You could also play safe and create a giveaway with region restriction in the first place.

You can write it in the description (it's not really enforceable though) but I don't think support will grant you a reroll, they'd most likely delete the GA if it turn out that it's region restricted.

With 1 of my GAs, even with region restricted and description to "Only X country", people from other countries in the same region won and I needed their permission to delete a GA they cannot win, after 2 tries I made friends only.

Don't worry about region and description, make a invite only and share with your friends, people don't read.