first choose a date, then click Optimize to get new parameters, then clear the date, and finally click Save. I am not sure if this is also an intentional behaviour.

Using the same parameters, can you press Evaluate with the date filled and cleared respectively and share the results?

The difficulty is 46% instead of 51%.

d = 46% is expected for these parameters and review history.

The conversion formula is

d = (D - 1)/9

where D is the difficulty from 1 to 10 and d is the difficulty in percent

Also, what was the reason for choosing to ignore some of the reviews during optimization? If the reason for ignoring the reviews is not good enough, it should not be a suprise that the algorithm works less effectively when some reviews are ignored.