in this case, the 2nd 264 field has to subfield $c, but only the 1st is included in the 2nd mapped field. I guess this could be fixed by adding a "entityPerRepeatedSubfield" instruction.
264 rule from mapping file:
"264": [
        {
            "rules": [
                {
                    "conditions": [
                        {"type": "remove_ending_punc, trim"}
                    ]
                }
            ],
            "target": "publication.place",
            "subfield": ["a"],
            "description": "Place of publication",
            "subFieldDelimiter": [
                {
                    "value": " ; ",
                    "subfields": ["a"]
                }
            ]
        },
        {
            "rules": [
                {
                    "conditions": [
                        {"type": "remove_ending_punc, trim"}
                    ]
                }
            ],
            "target": "publication.publisher",
            "subfield": ["b"],
            "description": "Publisher of publication",
            "subFieldDelimiter": [
                {
                    "value": " ; ",
                    "subfields": ["b"]
                }
            ]
        },
        {
            "rules": [
                {
                    "conditions": [
                        {"type": "remove_ending_punc, trim, trim_period"}
                    ]
                }
            ],
            "target": "publication.dateOfPublication",
            "subfield": ["c"],
            "description": "Date of publication",
            "subFieldDelimiter": [
                {
                    "value": " ; ",
                    "subfields": ["c"]
                }
            ]
        },
        {
            "rules": [
                {
                    "conditions": [
                        {"type": "set_publisher_role"}
                    ]
                }
            ],
            "target": "publication.role",
            "subfield": [
                "a",
                "b",
                "c"
            ],
            "description": "Role of publication, defined by 2nd indicator",
            "applyRulesOnConcatenatedData": true
        }
    ]
----------------------------------------------------------------------------------------
264 fields from MARC source:
{
    "264": {
        "ind1": " ",
        "ind2": "1",
        "subfields": [
            {"a": "Wakefield, MA :"},
            {"b": "Fesarius Publications,"},
            {"c": "[1988]"}
        ]
    }
},
{
    "264": {
        "ind1": " ",
        "ind2": "4",
        "subfields": [
            {"c": "©1988,"},
            {"c": "▒<84><97>1988"}
        ]
    }
}
----------------------------------------------------------------------------------------
instance output:
   "publication": [
        {
            "role": "Publication",
            "place": "Wakefield, MA",
            "publisher": "Fesarius Publications",
            "dateOfPublication": "[1988]"
        },
        {"dateOfPublication": "©1988 ;"}
    ]