kea dhcp option for debian preseed file

[cybermcm]

hello,
 I recently switched to kea and was able to use my debian preseed file with the following option in 'manual config mode'

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        "client-classes": [
            {
            "name": "deb-preseed",
            "test": "substring(option[vendor-class-identifier].text,0,3) == 'd-i'",
            "boot-file-name": "https://xyz.net/preseed.cfg"
            }
        ],
I noticed that it is now possible to set options via gui but it doesn't work for me.
I tried option 124 with hex code 642d69 for (d-i), see screenshot but if I set this option and inspect kea-dhcp4.conf it only shows

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"client-classes": [
            {
                "name": "22e436d9-2826-445b-ab48-d7b68058ca8f",
                "test": "option[124].hex == 0x642D69"
            }
        ]

Any ideas?