My first approach tries to reduce the false match of substrings, but it was not perfect yet, because each COMPOSER value needs a left and a right delimiter to give a distinctive value to compare.
As the delimiter character I proposed the $char(9), because it nearly never exists in a COMPOSER value, but you are free to apply some other seldom used character as a delimiter.
This filterstring works for me:
"$strstr('|'$meta_sep(COMPOSERS,'|')'|','|'%COMPOSER%'|')" GREATER 0
... or equivalent as a formatstring ...