Typically, kerosene-LOX engines run fuel-rich, with an oxidizer:fuel mass ratio of about 2.5:1 (as compared to 3.5:1 for complete combustion), leaving significant amounts of unburned carbon soot in the exhaust, which glows yellow as it combusts with oxygen in the atmosphere.
The oxidizer:fuel mass ratio for complete combustion of kerosene with hydrogen ...
Sutton edition 7 mentions them without too much detail
Jet vanes are pairs of heat-resistant, aerodynamic wing-shaped surfaces submerged in the exhaust jet of a fixed rocket nozzle. They were first used about 55
years ago. They cause extra drag (2 to 5% less Is; drag increases with larger
vane deflections) and erosion of the vane material. Graphite jet ...
I think Wikipedia's Bristol Siddeley Gamma; Advantages of kerosene / peroxide engines can shed some light on the missing light.
The combustion formula of kerosene and hydrogen peroxide is
and you can see that the exhaust is mostly water, which results in a clean, transparent flame and increases thrust ...
Partial answer for solid motors:
Fundamental Aspects of Solid Propellant Rockets (Williams, Barrere, and Huang, ~800 pages, 55 MB) says that if combustion products condense into solids inside the motor, the effect can be detrimental.
From Sections 3.2.6 (page numbered 78):
Typical results show substantial performance losses for particle diameters above 2 ...
Partial answer to get the ball rolling:
The RL-10B-2 and RL-10C-2-1 variants of the venerable RL10 LH2/LO2 upper stage engine feature an expandable nozzle with a 280:11 area ratio and an amazing 465.5 Isp.
Aerospaceguide.net RL10 article
Image source: Aerojet Rocketdyne
Image source: Impact of dynamics on the design of the RL10B-2 extendible carbon-carbon ...
Since this question was posted, plenty of exhaust plumes have been filmed from the ground giving a nice impression of the long term expansion behavior.
So, no, I wouldn't call it bulb shaped, but yes, the exhaust expands in ...