The heat haze shader effect doesn’t loop cleanly (i.e. ending state != starting state) so there is a “blip” in the animation.
Video showing this, in which I’ve tried Normal, Linear, and Straight modes (what is the difference between Linear and Straight?):
You were right, this is fixed.
Heat haze mixes two waves and the second one was running 1.3 cycles per
loop instead of a whole number. So at the end of the loop it sat about a
third of a cycle away from where it started, and that gap is the blip.
It also explains why Normal, Linear and Straight all behaved the same,
those change the speed curve but not the wave itself.
If you want to confirm it is the same thing I found, set Complexity to 0
and the blip should disappear completely, then come back as you raise it.
Fix is in for the next update.