I've observed a start-up overvoltage in LTspcie.
I've tried increase output capacitor, reduce load, delete LC filter etc.. But the overvoltage is always present.
I also see a small overshoot when circuit in LTspice is set to 12V. I suspect that the LTspice model is not perfect. The actual part doesn’t seem to have overshoot as shown in the data sheet.
Try to use the test jig of LTSpice for the simulation of LTM4661 and change the input and load conditions according to your needs. Kindly refer to page 8 of the datasheet for the output voltage programming…