A negative leap second means that 23:59:59 is skipped, you go from 23:59:58 to 00:00:00, which is monotonically increasing, in both UTC and unixtime.

Positive leap seconds are monotonically increasing in UTC, where you get 23:59:60 between 23:59:59 and 00:00:00, but not in typical implementations of unix time where 23:59:59 repeats, and with milliseconds you go (breifly) 58.999 -> 59.0 -> 59.999 -> 59.0 -> 59.999 -> 0.0

If you're only counting seconds, then both UTC and unixtime are always monotonically increasing.