EDIT

The web service returns one specimen for this taxon:

http://api.cybertaxonomy.org/caryo_nepenthaceae/occurrence/byAssociatedTaxon.json?relationshipsInvers=&relationships=&taxonUuid=fb987766-3ea5-45e0-a606-ca9b9d1253cf

{

    "prevIndex": null,
    "records": [
        {
            "data": [ ],
            "individualCount": "",
            "recordBasis": "PreservedSpecimen",
            "uuid": "8793b96f-8265-47f4-82a5-9e5e636cd8f3",
            "identityCache": "",
            "preferredStableUri": "",
            "catalogNumber": "",
            "barcode": "",
            "collectorsNumber": "",
            "created": "2018-06-02T09:15:00.000+02:00",
            "mostSignificantIdentifier": "",
            "originalLabelInfo": "",
            "titleCache": "Philippines, Luzon, Prov. Ilocos Norte, Burgos, male fl . 23 – 25 Nov 1916 Fenix BS 26726 (neotype: US, photo, K, isoneotypes: BO n.v., SING n.v.).",
            "protectedTitleCache": true,
            "updated": "2018-06-02T19:18:15.000+02:00",
            "exsiccatum": "",
            "protectedIdentityCache": false,
            "accessionNumber": "",
            "lsid": null,
            "class": "DerivedUnit",
            "publish": true
        }
    ],
    "suggestion": "",
    "count": 1,
    "lastRecord": 1,
    "pageSize": 30,
    "nextIndex": null,
    "currentIndex": 0,
    "indices": [
        0
    ],
    "pagesAvailable": 1,
    "firstRecord": 1,
    "class": "DefaultPagerImpl"

}

it is not shown in the portal, though.

The actual problem is that the webservice portal/taxon/fb987766-3ea5-45e0-a606-ca9b9d1253cf/associatedFieldUnits.json?&pageSize=50 is not returning a fieldunit.

This is not a bug in the webservice. The Specimen in question actually has no FieldUnit.
So this is primarily a data problem and also a problem with the compressed derived unit table, which can only display specimens which have a FieldUnit.
When the default specimen display is being used the Specimen-section is no longer empty:

Adding an empty FieldUnit can help to solve the problem of the empty section to some degree. But the result is not very appealing: