It’s not the official flag, it’s just the flag that everyone associates with Wales but there is no official Welsh flag. That’s just the one everyone’s ended up using but if you look it up there is no standard definition for what the Welsh flag looks like.
In 1959, after successful lobbying by the Gorsedd of Bards and others, Queen Elizabeth II made the red dragon on a green and white background the official flag for Wales.
It’s not the official flag, it’s just the flag that everyone associates with Wales but there is no official Welsh flag. That’s just the one everyone’s ended up using but if you look it up there is no standard definition for what the Welsh flag looks like.
https://www.bbc.com/wales/history/sites/themes/society/flag_officialemblem.shtml