As we are calculating the median CLVs across all cohorts (i.e. across months according to when someone first purchased), if newer cohorts are more valuable than older ones (for instance you increased the price 6 months ago), it will drag up the 3 month and 6 month CLVs before they can drag up the 12 month CLV (and it will drag up the 3 month CLV before it affects the 6 month CLV).
When those newer cohorts have actually been with you for 12 months, you will see the 12 month CLV increase such that the 12 month CLV > 6 month CLV.
However if you look at each individual cohort on its own (i.e. not an average across cohorts so just June 2021 for instance), the numbers are indeed cumulative and will not decrease for a given cohort.