ifelse with NAs not always evaluating as NA?

While NAs and ifelse have been discussed here before, I would like to understand a particular detail of this question, which has not yet been addressed before.

A reproducible example:

transflag <- c("YES", "YES", "NO", "NO", "NO", "NO", "YES", "YES", "NO")
class <- c("SC", "IC", "IC", "SC", NA, "IC", "IC", NA, NA)
prod <- c("P1", "P2", NA, "P1", "P2", "P1", NA, "P2", "P1")

data <- data.frame(class, prod, transflag)

data$transflag <- as.character(data$transflag )
data$class <- as.character(data$class )
data$prod <- as.character(data$prod )

data$transflag4 <- ifelse(data$prod == "P1" & data$class == "SC" , "NO", data$transflag)

data$transflag5 <- ifelse(data$prod == "P2" & data$class == "SC" , "NO", data$transflag)

How come that in transflag4 , the result is NA, while there is a result in transflag5 is defined? This is strange, considering that other posters mentioned that the evaluation of anything with NA is NA (recode variables with NAs with ifelse() in R)