I experienced the same turntable derailing problem when I created my recently released MCSA-1920s_LONG-DAYS-WORK suite of scenarios. In scenario #2, when I pushed a flatcar onto the turntable with the Vulcan, I had to include the instruction “Nudge it on to the turntable very gently and leave it there. Do not get the Vulcan's pilot wheels on the turntable. She’ll derail every time.”
This locomotive on this turntable, backing off on turntable lead #1 will always derail. The entire locomotive did not have to be on the turntable. Just getting the pilot wheels on the TT and then backing off would crash the game.