For some SAMs, missile speed suddenly is constant while the rocket is still burning.

[Mad_Shell]

While the missile is still burning, its speed suddenly becomes almost constant. I observed that on SA-2, SA-6, SA-10, SA-11 (perhaps more, didn't test them all).

A few videos below(too big to drop here, check missiles speeds at the bottom)

https://imgur.com/a/vHPZtCJ

[Northstar98]

Most likely a DCS-ism with the simplified flight modelling for SAMs, though this does seem a little odd.

[nighthawk2174]

2 hours ago, Northstar98 said:

Most likely a DCS-ism with the simplified flight modelling for SAMs, though this does seem a little odd.

Its this exactly, while some missiles will be quite steady irl with their sustain motor it won't be in most situations.  In DCS however the majority of SAM's have a super simple FM and as such they just accel up to a set speed and maintain that speed for a set time.