In addition to setting MAX_FANOUT, try setting ADV_NETLIST_OPT_ALLOWED to ALWAYS_ALLOW.
This can be set by adding the following to the qsf file
set_instance_assignment -name MAX_FANOUT 10 -to <node name>
set_instance_assignment -name ADV_NETLIST_OPT_ALLOWED ALWAYS_ALLOW -to < node name>
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category: Quartus® Prime
tool: Quartus® Prime
device: -to