My guess is Mendix just uses the first 'Item' element to read the maping. The first element in your example does not have an adress, only the second item does. Try importing with this:
{
"apiVersion": "2.0",
"meta": {},
"data": {
"itemsPerPage": 10,
"startPage": 1,
"totalItems": 2,
"items": [
{
"kvkNumber": "68727720",
"branchNumber": "000037143557",
"rsin": "857567433",
"tradeNames": {
"businessName": "Test NV Katrien",
"shortBusinessName": "Test NV Katrien",
"currentTradeNames": [
"Test NV Katrien"
],
"currentStatutoryNames": [
"Test NV Katrien"
]
},
"hasEntryInBusinessRegister": true,
"hasNonMailingIndication": true,
"isLegalPerson": false,
"isBranch": true,
"isMainBranch": true,
"addresses": [
{
"type": "vestigingsadres",
"street": "Rietdekkershof",
"houseNumber": "8",
"houseNumberAddition": "",
"postalCode": "9646AS",
"city": "Veendam",
"country": "Nederland"
}
]
}
]
}
}