Lubridate Package: Issue with date_time column

I can't reproduce your problem

library(ggplot2)
sample_data <- data.frame(
  stringsAsFactors = FALSE,
              Date = c("14-03-20","14-03-20",
                       "14-03-20","14-03-20","14-03-20","14-03-20","14-03-20",
                       "14-03-20","14-03-20","14-03-20"),
               Day = c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0),
              Time = c("00:00:30","00:00:30",
                       "00:02:30","00:02:30","00:04:30","00:04:30","00:06:30",
                       "00:06:30","00:08:30","00:08:30"),
               Sex = c("M", "M", "M", "M", "M", "M", "M", "M", "M", "M"),
         Treatment = c("CTRL","WT","CTRL","WT",
                       "CTRL","WT","CTRL","WT","CTRL","WT"),
           Perched = c(4, 2, 1, 1, 3, 1, 2, 3, 1, 1),
            Ground = c(1, 3, 4, 4, 2, 4, 3, 2, 4, 4),
              Feed = c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0),
             Water = c(1, 0, 0, 0, 0, 0, 0, 0, 0, 0),
         date_time = as.POSIXct(c("2020-03-14 00:00:30","2020-03-14 00:00:30",
                       "2020-03-14 00:02:30","2020-03-14 00:02:30","2020-03-14 00:04:30","2020-03-14 00:04:30",
                       "2020-03-14 00:06:30","2020-03-14 00:06:30","2020-03-14 00:08:30","2020-03-14 00:08:30"))
)

ggplot(sample_data, aes(date_time, Perched, color = Treatment)) +
  geom_line() +
  scale_x_datetime(breaks = "2 days")

Created on 2020-08-11 by the reprex package (v0.3.0)
Can you provide a proper REPRoducible EXample (reprex) illustrating your issue?