An increase of elongation of the SS316L sample by 5-8% indicates of the increase of the ductility of the sample, by the 1050 degree heat treatment.
The heat treatment of the SS316L around the crystallization remperature, will definetly anneal the sample to homogenized microstructure with better ductility than that of the as recieved sample.
The elongation in steel is affected by heat treatment and stress state after the onset of necking. If you are defining elongation as fracture strain then stress state will play some role.
Heat treatment have influenced in the grain restructuring. So, due to fine grain structure of SLM parts after Heat treatment, their ductile behaviour increase. This have also influenced On the elongation of SS.
You may also observe this increase in elongation due to Heat treatment in other alloys fabricated by SLM process Like AlSi10Mg.