I am also having this exact same issue with two Ultra II drives purchased directly from Amazon.co.uk warehouse (not a reseller). The first was purchased on 10 February 2016 and second on 4 March 2016.
Both drives have the latest firmware X41100RL and I have the latest Sandisk Dashboard 1.4.1
Dashboard reports both drives have Poor health due to SMART issue 232 Available Reserve Space. Value (Raw) 100, Threshold 4%.
I’m very disappointed to have two drives with the same issue but suspect it is more likely an issue with the Dashboard software.
It also has not helped my confidence in the company, as a first time Sandisk customer, that when I tried to register one of the drives, so that I could request an RMA, the registration failed with the error, “Invalid Product Code for this product”.