According to the documentation the valid values for ReaderConfig.startOffset (https://github.com/mostafa/xk6-kafka/blob/main/api-docs/docs/interfaces/ReaderConfig.md#startoffset) are the constants START_OFFSETS_FIRST_OFFSET and START_OFFSETS_LAST_OFFSET.

Those two constants are not included in the module-level constants (see console.log(kafka);) and therefor have no value set and do not work.
The module-level constants include the constants FIRST_OFFSET and LAST_OFFSET though (which work fine).