There is an interoperability cut in 6lo-over-bluetooth (Bluetooth's IPSP) that happened in Linux around 4.12 and in nRF SDK between the currently-used "nRF IoT SDK" and the "nRF SDK" (which supports it at least since the 14.1 release)

The current documentation in RIOT says it requires the old behavior by indicating the maximum supported Linux version as 4.12.

Please consider updating the upstream version, allowing to set BLE_6LOWPAN_LEGACY_MODE = 0, and updating the documentation.

(Note this is all from reading current documentation; I did not test the behavior as I don't have suitable hardware yet, so the fix might be as easy as stating in the documentation that RFC7668 mode can be enabled).