Of the 4 'fixed' tabs, the two at the front are not bonded correctly to the beams. It looks like contact set 138 and 148 were done 'two at once' attempting to bond both side tabs at once to the rail, and this isn't working for some reason.
To diagnose this I ran the simulation as provided. I then animated the displacement plot. The frame can be seen moving relative to the two fixed 'feet'.
Remove the front plates from their contact sets and add new ones between them and the rails. Everything looks good when the study is re-run without soft springs.