Using QSFP28 Module at Lower Lane Rate
The problem is often in clock-recovery circuits. The CDR chips used in these kinds of modules have PLLs/recovery loops with limited locking rates. An example (but made for QFSP40) is the Semtech GN2405A. As a result, you can only use them with symbol rates that match them. Sometimes, as is the case with the Semtech IC I linked to, you can have modules that can lock to lower rates too, but again only fixed windows. Sometimes you can also disable the retiming circuits of such a chip. However, all of this stuff is controlled by the microcontroller integrated in the QSFP module, and is not something you have access to as user.