How country flags are encoded
Most flag emoji for countries are pairs of regional-indicator symbols derived from two ASCII letters. That convention works for standard two-letter country codes, but not every geographic flag in Unicode is represented this way. Subdivision flags and special sequences use different encoding rules.
Two letters are not automatically a standard flag emoji
The conversion now rejects regional-indicator pairs that are not in the browser RGI flag set instead of presenting every A–Z pair as an interchangeable flag.