difference between two datetime displays wrong output

Question

I want to keep one record for every 3400 seconds to 3600 seconds, this chunk of code checks first row with the next row and if the id is the same it will then check if the time is 3400 or more secs apart, if it is it returns true, if not then it returns false. However, as you can see from the below output, it is returning FALSE from 12:00:23 to 13:00:09 (because the rows in between are false). Is there a way to amend it so it will give me my desired output (as below)?

TIA

Data:

id<-c('8959', '8959', '8959', '8959', '8959', '8959', '8959','8959',
'8959', '8959', '8959', '8959', '8959','8959', '8959', '8959', '8959', 
'8959', '8959', '8959','8959','8959', '8959', '8959', 
'8959', '8959','8959','8959','8959','8959')

t<-c('2021-08-14 06:00:53', '2021-08-14 07:00:10','2021-08-14 08:00:20',
     '2021-08-14 09:00:30','2021-08-14 10:00:10','2021-08-14 11:00:11',
     '2021-08-14 12:00:23','2021-08-14 12:10:09','2021-08-14 12:20:08',
     '2021-08-14 12:30:13','2021-08-14 12:40:13','2021-08-14 12:50:21',
     '2021-08-14 13:00:09','2021-08-14 13:10:08','2021-08-14 13:20:53',
     '2021-08-14 13:30:08', '2021-08-14 13:40:09', '2021-08-14 13:50:08',
     '2021-08-14 14:00:20','2021-08-14 14:10:08','2021-08-14 14:20:09',
     '2021-08-14 14:30:20','2021-08-14 14:40:15','2021-08-14 15:00:08',
     '2021-08-14 16:00:12','2021-08-14 16:26:18','2021-08-14 16:30:09',
     '2021-08-14 17:00:13','2021-08-14 18:00:53','2021-08-14 19:00:36')

library(lubridate)

oc.df<-as.data.frame(cbind(id,t))
oc.df$t<-ymd_hms(oc.df$t)

Chunk of code:

ind.data <- oc.df %>%
  group_by(id) %>%
  mutate(difftime = as.numeric(hms::as_hms(t) - hms::as_hms(lag(t, 1)))/3400) %>%
  mutate(independent = case_when(
    is.na(difftime) ~ TRUE,
    difftime >= 1 ~ TRUE,
    difftime < 1 ~ FALSE,
    TRUE ~ FALSE)
  )

Current output:

t                   id      difftime.     independent
2021-08-14 06:00:53 8959    -18.141785714 FALSE
2021-08-14 07:00:10 8959    1.270357143   TRUE
2021-08-14 08:00:20 8959    1.289285714   TRUE
2021-08-14 09:00:30 8959    1.289285714   TRUE
2021-08-14 10:00:10 8959    1.278571429   TRUE
2021-08-14 11:00:11 8959    1.286071429   TRUE
2021-08-14 12:00:23 8959    1.290000000   TRUE
2021-08-14 12:10:09 8959    0.209285714   FALSE
2021-08-14 12:20:08 8959    0.213928571   FALSE
2021-08-14 12:30:13 8959    0.216071429   FALSE
2021-08-14 12:40:13 8959    0.214285714   FALSE
2021-08-14 12:50:21 8959    0.217142857   FALSE
2021-08-14 13:00:09 8959    0.210000000   FALSE
2021-08-14 13:10:08 8959    0.213928571   FALSE
2021-08-14 13:20:53 8959    0.230357143   FALSE
2021-08-14 13:30:08 8959    0.198214286   FALSE
2021-08-14 13:40:09 8959    0.214642857   FALSE
2021-08-14 13:50:08 8959    0.213928571   FALSE
2021-08-14 14:00:20 8959    0.218571429   FALSE
2021-08-14 14:10:08 8959    0.210000000   FALSE
2021-08-14 14:20:09 8959    0.214642857   FALSE
2021-08-14 14:30:20 8959    0.218214286   FALSE
2021-08-14 14:40:15 8959    0.212500000   FALSE
2021-08-14 15:00:08 8959    0.426071429   FALSE
2021-08-14 16:00:12 8959    1.287142857   TRUE
2021-08-14 16:26:18 8959    0.559285714   FALSE
2021-08-14 16:30:09 8959    0.082500000   FALSE
2021-08-14 17:00:13 8959    0.644285714   FALSE
2021-08-14 18:00:53 8959    1.300000000   TRUE
2021-08-14 19:00:36 8959    1.279642857   TRUE

Desired output:

t                   id      difftime.     independent
2021-08-14 06:00:53 8959    -18.141785714 FALSE
2021-08-14 07:00:10 8959    1.270357143   TRUE
2021-08-14 08:00:20 8959    1.289285714   TRUE
2021-08-14 09:00:30 8959    1.289285714   TRUE
2021-08-14 10:00:10 8959    1.278571429   TRUE
2021-08-14 11:00:11 8959    1.286071429   TRUE
2021-08-14 12:00:23 8959    1.290000000   TRUE
2021-08-14 13:00:09 8959    0.210000000   TRUE
2021-08-14 14:00:20 8959    0.218571429   TRUE
2021-08-14 15:00:08 8959    0.426071429   TRUE
2021-08-14 16:00:12 8959    1.287142857   TRUE
2021-08-14 17:00:13 8959    0.644285714   TRUE
2021-08-14 18:00:53 8959    1.300000000   TRUE
2021-08-14 19:00:36 8959    1.279642857   TRUE