Hello,
using displacement controlled requires more care. First, from how I have written the demo, the Newton Raphson system is expressed on the displacement increment, so you need to add a DirichletBC with imposed value equal to the displacement increment over the time step and not the total displacement. Second, only the first iteration should have a non-zero DirichletBC all other iterations must be computed with zero DirichletBC, otherwise you will accumulate imposed displacements over all iterations and will not converge (see for instance: Custom Newton Solver problem with Dirichlet conditions)