Did you use any Command Objects to define spring constants?
Command objects don’t have units, so if you defined a spring constant in a command object, it could be given a value of 10 for the spring stiffness. When solving in mm, that means 10 N/mm, but when solving in meters that spring stiffness is 10 N/m.
Whenever I create a Command Object that includes an input parameter that has units, I use the Analysis Setting to set the Solver Unit System to Manual and set the Unit to what I want. That way the Unit setting in Mechanical doesn’t change what the solver does.