I have never used \K - feel free send a PR as it seems you know how it needs to behave

Citing the php docs:

\K can be used to reset the match start. For example, the pattern foo\Kbar matches "foobar", but reports that it has matched "bar". The use of \K does not interfere with the setting of captured substrings. For example, when the pattern (foo)\Kbar matches "foobar", the first substring is still set to "foo".

ref: https://www.php.net/manual/en/regexp.reference.escape.php

php repro: https://3v4l.org/PcEOS

To fix this issue, $matches[0] must be of type string (instead of non-empty-string) when \K escape sequence is present anywhere in the regex. Or only when "there is NOT at least something always matched after \K - a\K.* vs a\K.+".

Looking into the code it can be implemented in https://github.com/phpstan/phpstan-src/pull/3897/files#diff-2d99eb3989780111d33d57f098cc862bed1aa0da5984be26570e4d0d2304c091R106.