In this case, I think the error is more clear. The mapping process is simply cutting off the formatted contents note after the 1st 3 subfields are processed. The result that you want is to have the entire field processed and concatenated together, which is what I would happen. thought 
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505 rule
"505": [
        {
            "entity": [
                {
                    "rules": [
                        {
                            "conditions": [
                                {
                                    "type": "set_note_type_id",
                                    "parameter": {"name": "Formatted Contents Note"}
                                }
                            ]
                        }
                    ],
                    "target": "notes.instanceNoteTypeId",
                    "subfield": [
                        "a",
                        "g",
                        "r",
                        "t",
                        "u"
                    ],
                    "description": "Instance note type id",
                    "applyRulesOnConcatenatedData": true
                },
                {
                    "rules": [
                        {
                            "conditions": [
                                {"type": "trim_period"}
                            ]
                        }
                    ],
                    "target": "notes.note",
                    "subfield": [
                        "a",
                        "g",
                        "r",
                        "t",
                        "u"
                    ],
                    "description": "Formatted Contents Note",
                    "applyRulesOnConcatenatedData": true
                },
                {
                    "rules": [
                        {
                            "value": "false",
                            "conditions": []
                        }
                    ],
                    "target": "notes.staffOnly",
                    "subfield": [
                        "a",
                        "g",
                        "r",
                        "t",
                        "u"
                    ],
                    "description": "If true, determines that the note should not be visible for others than staff"
                }
            ]
        }
    ]
------------------------------------------------------------------------------------------
505 MARC source:
    "505": {
        "ind1": "0",
        "ind2": "0",
        "subfields": [
            {"g": "Side one:"},
            {"t": "Star sisters /"},
            {"r": "(Jean Stevenson)"},
            {"g": "2:38 --"},
            {"t": "Ladyhawke! /"},
            {"r": "idea by Roberta Rogow"},
            {"g": "3:30 --"},
            {"t": "Go, traveller /"},
            {"r": "(lyrics, Jean Stevenson ; music: \"O\" Magnum Mysterium\", Tomas L. de Victoria)"},
            {"g": "2:55 --"},
            {"t": "Dairy Queen /"},
            {"r": "(Lyrics, Barbara Higgins ; music: \"Come Ye Knights\", Sheila Willis)"},
            {"g": "2:58 --"},
            {"t": "Robin Hood /"},
            {"r": "(T.J. Burnside)"},
            {"g": "2:30 --"},
            {"t": "Arafel's song /"},
            {"r": "(lyrics, Mercedes Lackey ; music, Kathy Mar)"},
            {"g": "4:40."}
        ]
    }
},
{
    "505": {
        "ind1": "0",
        "ind2": "0",
        "subfields": [
            {"g": "Side two."},
            {"t": "Wolf and hawk /"},
            {"r": "(Sheila Willis)"},
            {"g": "5:40 --"},
            {"t": "Challenge /"},
            {"r": "(Bill Roper)"},
            {"g": "3:00 --"},
            {"t": "Wishful thinking /"},
            {"r": "(lyrics, Peter Thiesen ; music, Peter Thiesen/traditional) --"},
            {"t": "Jaq's song /"},
            {"r": "(counterpoint lyrics, Linda Melnick)"},
            {"g": "2:50 --"},
            {"t": "Dreamer's lament /"},
            {"r": "(T.J. Burnside)"},
            {"g": "4:10 --"},
            {"t": "Technical difficulties, part III /"},
            {"r": "(lyrics by Technical Difficulties ; music, \"Canon in D\", Johannes Pachelbel)"},
            {"g": "3:05."}
        ]
    }
}
------------------------------------------------------------------------------------------
505 instance output
{
    "note": "Side one: Star sisters / (Jean Stevenson)",
    "staffOnly": false,
    "instanceNoteTypeId": "5ba8e385-0e27-462e-a571-ffa1fa34ea54"
},
{
    "note": "Side two. Wolf and hawk / (Sheila Willis)",
    "staffOnly": false,
    "instanceNoteTypeId": "5ba8e385-0e27-462e-a571-ffa1fa34ea54"
}