My point is you didn't say what was wrong or how it was wrong or which part of the information was accurate or inaccurate. And while most people may not walk at 5.5 + miles an hour you said "I've walked the same route countless times...", without other information one might infer that you are a regular walker at which point one may also infer that you are proficient at walking and conclude that a proficient walker is perhaps capable of achieving a near 10 min mile. I included the numbers not to teach but to draw more information since the given data supports the final number of 4.35 miles traveled with no comparison to the distance the was actually covered.
Sorry if I don't have your device, but I do have a device with 4.4 and a GPS and issues/bugs do sometimes cross devices with OS updates. Now that you've included how the information is incorrect I will say that, and again I don't say this to teach just as information to be considered, GPS units are finicky and GPS readings can be affected by tree cover, tall buildings, and overcast skies. If the trees in your area have started to leaf or the weather there is, as it has been here the last couple of days, overcast and rainy external influences may be the cause of the inaccuracies. Have you used the app prior to this walk but after receiving the update? Was it accurate then or has this been an issue since the update?