Apple watch series 6 VO2 max inaccuracies

My Series 6 watch correctly reported my VO2 max as about 28 in Nov 2020. Since January 2021 my VO2 max has been just under 18! I exercise daily with over 30 minute brisk walks. I checked out my VO2 max using a Garmin Vivosmart 4 for several days and it reported my VO2 max as 35 my Watch reported 17.9 for the same period. I guess my Apple watch has an issue! Anybody have any ideas?