My car is also on the Bess. I've been watching it for a few days. When it first left Bremerhaven it did something similar but I don't think it's an indication that something is wrong. It has, so far, met all of the projected port arrival times within a few hours (Zeebrugge and Southampton).
Perhaps they are doing testing, training, or avoiding storms.
(Maybe this should go in the Order Tracking forum.)