FIT Budgeting occurs when a item consists of more than one system. It is very similar to apportion of Target level of safety(TLS) in avionics domain. FIT budgeting is related to division of random hardware failure rate for some systems. If more than one supplier has to develop, it will not be easy.
Regarding ASIL decomposition, decomposed QM means that the decomposed requirement do not have to develop according to ISO 26262, but FIT budgeting is still valid. Total FIT number is not changed even though you decompose safety requirements.
In this case, the system which is allocated QM(original ASIL) has determined to be no risks in the systematic aspect.
If this policy follows similarly in the avionics domain, a subsystem which is included in the high SIL systems but is not allocated to safety requirements, it might be okay develop as QM. Of course, FIT budgeting (I mean, apportion of TLS) needs to be considered. I’m not sure, it is my guess.