The script functions for multi-value variables work by converting the multi-value variable to a string (using the MULTI_VALUED_JOINER) and then splitting it again.

If any of the values of the multi-value variable contain the MULTI_VALUED_JOINER character(s), then the script will not work correctly i.e.

%_myvariable% = ['Surname: Vivaldi; Forename: Antonio', 'Surname: Bach; Forename: Johann']

Then e.g.

$inmulti(%_myvariable%,Surname Vivaldi; Forename: Antonio') will be false rather than true.

The same is probably true for some of the other multi-value script functions (e.g. $copymerge).

Note: I am setting this to low priority as the likelihood of this occurring with MB-supplied multi-value data (such as the Artists tag) is low.