This is a long-standing bug on the 255 - I've only ever seen an excessively high threshold speed on this screen. The actual value is shown below which I got from the VO2MAX screen:
I had pace/speed preference set to SPEED so have now changed it to PACE so will see if this screen works for pace to confirm this is a bug with speed.
( update 16th April )
You can see that when PACE is the preferred speed/pace setting there's no issue :
