Current source with base resistance
In order for Q6 to operate as a CC source when hFE drops rapidly below Vce=2V, the base current must be increased to at least 3% of Ic.  When Vce=Vce(sat) the hFE is only 10 or base current of 10%.  BUT, Q6-b input ESR or incremental impedance drops to the low base spreading resistance when saturated, and this could bypass some of the emitter current in Q7 and increase the current source value near saturation, thus there are limits to Rb for min and maximum.

The ratio of Rb to Re (current sense) depends on high hFE devices but a ratio of 33:1 is a good ratio.