I had same problem too, on about weeks ago.
try this one, it fixed the bug on me.
If it work normally, this should be enough, you'll see: "Squad original plume" on original SRB (BACC/Hammer/Kickback), and plume with Apoapsis Motor.
if didn't, keep first one, add
Restock Waterfall Expansion 3.0.0
and
RSMP ( Solid_Rockets_Waterfall_Effects-3.0_RSMP )
https://spacedock.info/mod/3051/Solid Rockets Waterfall Effects
that's my config (for now)
Restock Waterfall Expansion 3.0.0 will cause BUG ("continue show Flames and generate Sound after Flame-Out", rfried May,1 posted)
I tryed BLUAV8R Fix, copy-paste and didn't work, I'll keep tinkering this _(:з」∠)_ Other times, when I'm less that busy.