You are correct, NTP prefers to jump first (if needed) and then slew afterwards (which is exactly what we want!), although it can jump again if the offset is too large.

In our case the jumps where because we also have PTP disciplining the same system clock, when you have both PTP and NTP fighting over the same clock, you will see jumping with the default settings.

For us it was easier to just do a one time NTP sync at the beginning/boot, and then sync the robots local network with only PTP afterwards.

Pedantically, a monotonic function need not have a constant first derivative. To take it further, in mathematics it is accepted for a monatomic function to have a countable number of discontinuities, but of course in the context of a digital clock that only increments in discrete steps, that’s of little bearing.

But that’s all besides the point since most sane time sync clients (regardless of protocol) generally handle small deviations (i.e. normal cases) by speeding up or slowing down the system clock, not jumping it (forward or backward).