It looks like fill isn't respecting groupings in the following
df <- data.frame(id="X", Month = 1:3, Year = c(2000, NA,NA))
dfa <- data.frame(id="Y", Month = 1:3, Year = c(NA,2001,NA))
group_by(rbind(df,dfa),id) %>% tidyr::fill(Year) %>% as.data.frame()
id Month Year
1 X 1 2000
2 X 2 2000
3 X 3 2000
4 Y 1 2000 <<< s/b NA
5 Y 2 2001
6 Y 3 2001
It looks like fill isn't respecting groupings in the following