Official comment
Hi Anthony, I am sorry you're experiencing this behavior from your SP-16 and I thank you for the demonstration video.
I've replicated this scenario and yes, there is indeed an issue with the unit arbitrarily assigning a note length of 64 to certain steps when manually entering in notes from the pads onto a MIDI track. Unfortunately, it is not possible to downgrade - I have conveyed this to the engineers as a bug. Let me get right back to you and let you know if there is anything more I can do.