i've tried a litte bit.

My findings can be broken down into the following:
It's true JS can is not able to understand (?i) it can only be used globally. Therefore:

console.log(/(?i:[A-Z]{3})/.test("abcdefghijkl")); 
// should return an error 
// or false if (?i) is ignored

console.log(/(?i:[A-Z]{4})/.test("abcdefghijkl")); // same here

If {x} is between 0 and 3 it returns true otherwise it returns false this is true for firefox nightly, firefox, brave, opera, chrome and edge.

The reason why I believe this is a bug:

After using safari it throws an error

Invalid regular expression: unrecognized character after (?

Therefore I believe you are right this is a bug in chromium and firefox.

I hope this helps and if anything is unclear or incorrect please let me know! :)