Seems to work mostly fine: https://community.fs.com/blog/40g-qsfp-to-10g-sfp-configuration-guide.html

Considerations for QSFP+ to SFP+ Split

Usually, this split configuration is supported on most switches provided by different vendors regardless of standalone and stacked ones. Certainly, you still need to check this function in the instruction of your switch or consult your vendor, as it’s the most basic condition of splitting QSFP+ to SFP+ configuration. Sometimes, most ports on a switch support split, but some don’t. In other cases, when a switch is deployed as a leaf switch, some ports do not support the split. Or when the port is already used as a stack port, this may also limit split configuration. Thus the conditions can be quite different based on your applications.