I'm helping rockcavera who first noticed parts of this.

The .pos property for $regml() and $regmlex() are not reporting the correct position when the string contains codepoints above 127.

Code:
//var %msg multibyte character $chr(233) test | echo -a pos: $pos(%msg,test) | noop $regex(%msg,/(test)/u) | echo -a regex: $regml(1).pos

In 7.52 and earlier

returns whether or not /u modifier is used:
pos: 23
regex: 23

In 7.53 and newer

returns whether or not /u modifier is used:
pos: 23
regex: 24

I then checked to see if $regmlex().pos did the same thing, and it does the same as $regml() in both versions. However at first I was having trouble avoiding an error message until discovering that $regmlex() wasn't working without using the <name> parameter even though /help indicates that [name] is optional.