gshift gives wrong results in subsetting queries #5962

          I encounter this problem too with the following example:

library(data.table)

dt <- expand.grid(
  date = 1:10,
  time = 1:10,
  id = 1:5
)

setDT(dt, key = c("date", "time", "id"))
dt[, x := runif(.N)]
dt[time == 1, x1 := shift(x, type = "lead"), by = id]

Key: <date, time, id>
Index: <time>
      date  time    id         x        x1
     <int> <int> <int>     <num>     <num>
  1:     1     1     1 0.1217285 0.2041882
  2:     1     1     2 0.2499479 0.3632787
  3:     1     1     3 0.9992822 0.3553096
  4:     1     1     4 0.9851168 0.1797294
  5:     1     1     5 0.2273191 0.2967562
 ---                                      
496:    10    10     1 0.2503363        NA
497:    10    10     2 0.2226453        NA
498:    10    10     3 0.3320792        NA
499:    10    10     4 0.3255982        NA
500:    10    10     5 0.1976716        NA

> dt
Key: <date, time, id>
Index: <time>
      date  time    id          x                                                                    x1
     <int> <int> <int>      <num>                                                                <list>
  1:     1     1     1 0.74905819 0.59769795,0.30605044,0.74046541,0.08694826,0.24060779,0.89059279,...
  2:     1     1     2 0.51506472 0.59769795,0.30605044,0.74046541,0.08694826,0.24060779,0.89059279,...
  3:     1     1     3 0.82395315 0.59769795,0.30605044,0.74046541,0.08694826,0.24060779,0.89059279,...
  4:     1     1     4 0.81986857 0.59769795,0.30605044,0.74046541,0.08694826,0.24060779,0.89059279,...
  5:     1     1     5 0.78196531 0.59769795,0.30605044,0.74046541,0.08694826,0.24060779,0.89059279,...
 ---                                                                                                   
496:    10    10     1 0.53337552                                                                      
497:    10    10     2 0.03719923                                                                      
498:    10    10     3 0.73477434                                                                      
499:    10    10     4 0.03107033                                                                      
500:    10    10     5 0.72256881