ANSWER!: Don't dump the rest of the packets you receive from one read. (There can be a lot more in it!)
Check your entire message size and move the rest back into the processing buffer.
Packets have been moving all the time right under my nose.
[SINGLEPLAYER (didn't test in MP)]
Writing in regards of InSim ISP_PLL(23) packet not being sent sometimes.
It works in one case and not the other.
Case 1) It works when the player is still in the screen which appears after you press Singleplayer.
By clicking the "Join Race" button and then "Spectate" after that
Case 2) It does not work when the player is already driving and uses Shift+S or selects it via escape menu.
It is not clear to me if the player slot should be preserved when doing Shift+S while driving or not.
If the player slot is not meant to be reserved then it should send the packet.
Further testing proved that each time the player does Shift+S and Shift+J(Join), the player slot changes but ISP_PLL is not sent.
Or I have read it wrong and the concept of "player leave (spectate - loses slot)" is conditional
This is great. Honestly, great.
The detail of weight transferring over to the inner steering wheel and outer rear wheel when steering distributes the weight evenly across rear wheels when in corners.
This also in theory gives the vehicle more max angle than just "60°" due the outer rear wheel getting more grip and 'holding it back' a little better than steering alone.