While testing a mod-mission I noticed that the thruster glows on one of my capship was completely static (as in: none of the usual flickering of the normal and tertiary glow sprites).
At first I thought I had broken something in the tables, but rotating my fighter around at full speed while in external view, I noticed the flickering animation comes and goes as you rotate the ship.
I turns out that ships oriented along the x-axis (both in + or - direction) show completely static thrusterglows when moving.
As the ship rotates away from the x-axis towards the z-axis, the flickering gradually starts. Ships pointing along the y-axis (straight up/down) also have static glows.
You can also see this in the F3 lab, enable thrusters in the rendering options and rotate any ship 90° clockwise/counterclockwise form the default position.
I checked in FS2 retail, and thrusterglows do the flickering animation consistently, independent of model orientation.
Retail FS2 even does the flickering at all times (i.e. when the ship is stopped), while fs2_open seems to scale the flickering intensity related with ship speed? So some part of the code that controls the flickering was changed at one point.
I also checked older builds in my folder, from fs2_open_19 to fs2_open_23, and all behave the same way. So if this is a bug, it's an old one.
See attached mission to test yourself ingame. Download