SpringJoint2D sets 'frequency' to 0 and 'dampingRatio' to 1 when changing values via script. For example in my script included in this folder, I try to set 'frequency' to 2 and 'dampingRatio' to 0.1 but the values which actually appear are 'frequency = 0' and 'dampingRatio = 1'.

1 Create new project
2 Drag scene "Spring2D_Bug" and script "SpringCheckr" to project (from attached folder)
3 Open scene "Spring2D_Bug", select Main camera in Hierarchy and attach script "SpringCheckr" to it
4 Press play and select Cube (that has Spring Joint 2D element) - notice that 'frequency = 0' and 'dampingRatio = 1' but should be 'frequency'=2 and 'dampingRatio'=0.1 according to script "SpringCheckr"