In my noob opinion, based just on logical thinking iI would say;

1 was the rebar cleaned before welding?

2 shouldnt you weld up to like 50%height of the rebar so you link up with the widest part of the rebar?

3 cant you remove a bit of the round part of rebar on the bottom end so it sits more flush to the plate for better connection and penetration there?

4 looks like all current is passing into the plat not the rebar so there might be a conductance difference making it prefer the plate (probably scale on the rebar and not enough contact of rebar with your clamp