Code:
//echo -a $regex(éa,/(*UTF8)/g)
returns -11:
Originally Posted By: pcre.txt

PCRE_ERROR_BADUTF8_OFFSET (-11)

The UTF-8 byte sequence that was passed as a subject was checked and
found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
value of startoffset did not point to the beginning of a UTF-8 charac-
ter or the end of the subject.
mIRC is probably advancing the offset to 1 instead of 2 in this case

It seems like this bug goes back to this fix/thread, I don't have an older version than 7.22 at hands so I only tried up to 7.22 (6.35 behaves correctly), which has the issue, and 7.21 has a fix for the original reported issue
Originally Posted By: 7.21
5.Fixed $regsubex() bug when handling empty substrings.

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